Historically locomotives had brakes which were operated separately to those on the train.
A proportional valve was fitted which when then controlled the brake application on the locomotive.
A locomotive on its own could not stop a train at full speed, indeed the locomotive brakes were normally very poor which was why they were restricted below line speed when running light.
Originally when a locomotives was shut down it became a "swinger" in that it had no brakes, only through pipes for the vacuum/air brake, and thus it required a "rider".
"Multiple unit" working did not result in any problems because the locomotive control pipes/cables were coupled between them but it did mean that only locomotives with compatible systems could couple. Those which had incompatible system could run but had to be manned either under power or if dead with a "rider"
That is the origin of Blue Star, Red Circle, etc coupling codes on locomotives.
Following an accident at Dorridge, when a failed train was propelled into the loop but ran through the stops at the end because the Driver could not control the train brake, a modification knoiwn as AFT )Assistance - Failed Train) was fitted. This was used when the train was assisten in rear and which allowed the Driver on the leading locomotive to make a brake application.
Obviously matters have moved on since those times, but it explains why locomotives were never hauled around at the rear of trains until relatively recently.
During the 1970s and 1980s the OHL construction trains on The Midland Suburban Electrification Scheme were specailly authorised to run with a locomotive at the rear under special operating instructions.
