Hello Allan
Sorry this has to be long, but it is all co-relative and fits like jig saw puzzle pieces to give a total picture --here goes.
If I recall correctly from an old obscure tech story about Hi-V and MUDC retrofits,
The Pelham seemed to always have Hi-V cars on it - and the doors when all cars had manual doors --could be controlled separately, independently.. The center doors had a separate lever to operate a "remote" air-valve operated center door mechanism. Of course, the end doors at the vestibules still had the "manual linkage doors.
At City Hall (and South Ferry before gap fillers)- the center door (remote air piston valve door control) levers would not be yanked.
Now a little more tech background on the door controls.
ALL Hi-V Cars built for the subway (IRT) from Gibbs (1904-5) and Composites (1902-3) had end doors only as built, manually operated by the twin levers at both side of the storm door location on the end of each car. The 1906 Deck roof cars similarly.
When center doors were cut into that earlier class of cars , a mechanical-pneumatic (air piston controlled) operating mechanism separately controlled the center doors, This smaller lever, and its downward connecting rod from the tiny reduction-gearbox at the lever end, was located under each end "storm window" of the vestible end outer sheet walls.
So, from this point (1908-10 center door installations to Composites, Gibbs, Deck Roof Hi-V cars---the center doors were NOT opened on any sharp curves (South ferry, City Hall) but at SOUTH FERRY ---ONLY the center doors were opened at the INNER LOOP where the platform edge did (and still does) have a solid masonry wall with "portal openings" lined up for the center doors ONLY on the standard IRT pre-war fleet. The center doors were the ONLY doors close to the platform on that long closed inner loop track & platform...reverse scenario of City Hall loop!
A "door guard or gateman" had to stand between each 2 cars and operate the end vestibule levers of each car next to his right and left-- as he stood between the cars on his stirrup steps, but he only operated ONE lever of the one center door...to his left... facing the front of the train. Therefore when the guard between the lst & 2nd car opened his doors, he opened the center door (small control lever) of the first car to his left, and the end vestibule doors of both the lst and 2nd cars cars he stood between.
The next guard did the same, opening the center door (small control lever) to his left for the 2nd car and both manual end vestibule doors (via the large levers) for car 2 and 3....and so on thru a 4,5, or 6 car train.
THE REARMOST GUARD (between the last 2 cars - opened both the end vestibule manual control door sets of his two adjacent cars he stood between, but also opened the left AND right (rearmost) car's center doors by manipulationg BOTH cars small pnuematic center door small control levers on the ends of both cars he stood between.
IN THE EVENT that ONE OR BOTH CARS of his cars were still in the tunnel (ie" short station..as most rearmost cars were expected to be in such situations) he had the options of deciding which doors to opoen or not on both cars, conditions relevant to same.
NOW --WE COME TO EARLY DOOR FORMAT OF IRT SUBWAY - SEMI AUTOMATIC DOORS -- but not yet full MUDC.
In 1915--an order of Headley-Body Hi-v cars came equipped with both the older style end "large lever controlled manual end vestibule doors" and "factory built" Center doors with the pneumatic small lever design - all hardware similar to that on earlier 1902 thru 1906 cars. Another batch of Hi-V cars (motors & trailers) came with an ALL mechanical-pneumatic door control system...but NOT MUDC type.
This was a primative fore-runner to the later retrofitted electro-pneumatical "MUDC" system retrofitted on many Hi-V's, all early Low-V's and factory-new-installed on the newest Low-v's from the factory (1920-22-24 models)
This early "automatic" door operation eliminated the brutal door opening levers on each side of the storm door. We can imagine they caused a number of back and shoulder sprains, injuries, etc.
Look in Gene Sansones Book --Evolution of the N. Y. Subways" Page 41, bottom photo, for a picture of this system..on a High-V trailer as new in 1918.
There was now a new system of two very small levers now located under the end storm windows, where the original single small pneumatic center door lever was located, and with two rods going down the exterior end sheet(s) and thru two holes in the conductor stirrup step plates, thru the anticlimber and to the car underside.
One small lever pneumatically controlled both end vestibule doors on that carbody-side of that car, the other small lever pneumatically controlled only the one center door on that carbody side of the that car.
This did have a small labor savings...ONE guard could now automatically, pneumatically operate ALL side doors on a pair of cars - three doors on the car to his right and three to the car on his left, with a small pair of levers to his right and left sides that he could yank with his two wrists, one set simultaneously at a time.
HE COULD ALSO CONTROL THE CENTER DOOR INDEPENDENTLY ---just as he could do on the early "battleship" cars - those having the "mechanical" large levers on end vestibule doors and pneumatic small levers for center doors.
This pneumatic system meant that in a 6 car train, you had a man (guard) between cars 1 & 2, (*) between 3 & 4 and (*) between 5 & 6. The (*) represent where a guard would be eliminated, but not so if these 6 cars all had the old manual type straight-mechanical "end-vestibule-large-lever-operated" doors.
There were still trains sets that used a full manual control vestibule door car type at each end of a 5 or 6 car train, using pneumatic door cars in the center of the consist. Likewise, a 5 or 6 car consist of all manual end vestibule doors, having no pneumatic door cars in the consist.
IN EITHER TRAINSET TYPE SCENARIO --the center doors were ALL still separately. individually controlled....for situations like South Ferry or City Hall Loop
OK - so now you see that these type cars ran on lines that used the curves at City Hall & South Ferry loops, and that under this fleet using manual mechanical and manual-pneumatic, thru the mid 1940's, you could control center doors separately from end doors..on all car classes from 1902 thru 1920 built.
ENTER Full-type-MUDC-control ---mid to late 1920's
The IRT struggling with the 5 cent fare, wanted to cut labor more. The MUDC suystem, used on other company systems & lines, was finally adapted to the IRT. The newest Low-V cars (1922-24) came with it--having Electro-Puenuatic door controls.
ALL doors controled at same time by a simple set of open and close buttons..no selectivity available.
The IRT used a similar system to convert about 450 of its post 1902 class wooden EL Gate cars to MUDC with enclosed ends where the gates and open-platforms were originally located, and outside hung 33" wide sliding doors, creating new enclosed vestibules.
The IRT converted, gradually, many of its Gibbs cars to full MUDC, installing newly built side end-vestibule side doors with a different (2 horizonal windows) window design. The end storm doors were ALSO replaced by a new "standard look" door sliding into a 2-wall- door-pocket (standard IRT design) this new type replacing the "hinged" swinging door as originally built.
Many of the 1910-1915 Headley Body Design Hi-V cars (those built NEW with their center doors---built just after the Deck Roof class cars) --those with the 3 panels in the end vetsibule and center doors --were converted in the mid to late 1920's, to MUDC - having their levers all removed and the MUDC boxes identical to those we see on todays Low-V museum set - installed.
The 50 deck roof motor cars were NEVER converted to MUDC --and spent their lives in the MUDC period as Pilot Motor cars at front and rear of locals and expresses...still having their totally manual-large lever econtrolled vestibule doors and the pneumatic small single lever for the center doors.
Many Hi-V Motor cars having the 2 panel (Gibbs, Deck Roof) and 3 panel (Headley Body cars) side doors would end up with a center door replacement retrofit with one single panel door of the Low-V style.
Hi-V Trailer cars had bodies and door styles (large single panel) like the Low-V Motor and trail class.
So, by the time City Hall Station closed - when was that, 1945, the IRT used cars only with manual-mechanical or manual-pneumatic door controls cars in trainsets --so to control center door operation independenly.
By 1946, all IRT cars still NOT having MUDC (Multiple Unit door control, as we know in use on cars using one conductor to operate all doors) ran as usual until scarpped in the 1951- 1959 period.
The first of these to go did so when the IRT 1918-1922 built Steinway low-V and 1939 built Worlds Fair Steinway class low-V units were replaced in 1949-50 on the Flushing (#7) Line by IRT R-12, 14, & 15 SMEE cars. Steinways went to Pelham mainly, where they replaced a number of non MUDC Hi-V Cars.
Of course, the center doors on all MUDC Steinways were not a problem at City Hall Station- the loop was already earlier closed to passenger use...and gap fillers had long been installed elsewhere at curved "problem platform" areas.... since the early 20's
Some of the non-MUDC class manual-mechanical lever controlled end-vestibule door High-V's Motor cars after 1946 were comnverted with MUDC trainlines so they could be used at ends (as pilots) and mid-train positions as needed with manual doors and pass MUDC electro-pnuematic door control circuits to sections on either side of them (mid train use) as well as Motorman MUDC door control indication light circuits. These were called "Modified" Motor Hi-V's, and of course still only ran with HI-V only class cars --MUDC cars or manual door cars.
So, by the period after 1946, MUDC was the rule on the IRT subways, Hi-V or Low-V type cars. Cars with all-Manual vestibule doors were utilized not only at both ends of HI-V MUDC-sets as "Pilot Motor cars" but also as used in 2 to 4 car shuttle services ---no MUDC or automatic "pneumatic" operation available in their lashup (except individually activated mech-pneuatic center doors)---ie: POLO Ground to Jerome-167 st.: South Ferry to Bowling Green: Dyre ave- 180th St.: Grand Central to Times Square, and etc.
The last bastion of the Hi-V MUDC and Hi-V "Modified Motor" mechanical door cars between 1950 and their passenger disuse around 1959-60, was the Pelham # 6, (serving separately with Steinway trainsets)
and Broadway (#1) Local and White plains Rd - Broadway-7th Avenue west side express.
Many HI-V Miodified motors were used in work trains until mid 1962 ---and replaced by Low-V's in that work service -- the Low-V's themselves being replaced in passenger use by the newest mainline IRT R-types.
So, now you can see that " true MUDC" IRT pre-war cars were not used on the City Hall Loop in passenger service as their center doors could not be independently operated separately from the end vestibule doors, and up until City Hall Loop Station closed, there were plenty of non-MUDC but semi-automatic "pneumatic-control-door" Hi-V cars as described way above, and manual-control-door cars, to allow customized selectivity of center door control in trainsets used servicing that station.
Hope this answers your question - Alan --tho quite long -its not a simple discussion of fact & history..its all co-mingled and related.
Regards - Joe