Going back to the late 1950s, there were four Nassau loop rush hour routes. Two of them, the Culver Express and the West End Short Line (or West End Local) appeared on the regular service guides. The other two, serving the Brighton and 4th Ave. lines, did not, and probably qualified as "Bankers' Specials".
In those days, all of these routes used BMT Standards, plus some ex-SIRT cars on the Culver. The 4th Ave. and West End services operated northbound via the Nassau loop (tunnel to Manhattan, bridge to Brooklyn), and the Brighton and Culver services southbound. The Culver and 4th Ave. trains used the switches north of DeKalb to get from the bypass tracks to the tunnel, the West End and Brighton specials stopped at DeKalb (although, I understand that prior to the Mid-1950s, there was a connection from the bypass tracks to the Brighton line south of DeKalb which allowed the Brighton Bankers' specials to bypass DeKalb.)
-- Ed Sachs