I used to commute (pre-pandemic) between Derby and Willington. When the train was formed of 2 x 2car 170s, the announcement was made at Derby that "only the front 3 coaches will be platformed, the rear coach doors will not be unlocked at Willington", so that passengers would have to walk through to the front car of the rear set.
I seem to remember that on one occasion, the train was formed of a 3-car at the Birmingham end and a 2-car at the Derby end. I knew that this would cause problems so I made sure I was in the 3-car set. The announcement about "front 3 coaches only" was made after the train had left Derby (so too late to get from one unit to the other), so I hope anyone over-carried was given a free taxi from Burton back to Willington, or a special-stop on the next northbound service (but make sure you board at the front of the train at Burton!)
Sorry - off topic relating to both Voyagers and University.