I'm using https://mods.factorio.com/mod/MultipleUnitTrainControl, and https://mods.factorio.com/mod/BatteryPack (my mod) to have a double-ended shuttle train that runs on sunshine stored in batteries, but while the train arrives when I call for it, the shuttle train UI doesn't show when I enter the train.

The vanilla train replacement provided by Multiple Unit Train Control works as expected, but the battery-powered train that I upgraded to does not; any ideas on what might be going wrong?

I added a second shuttle train module to the other end of the train and it started working again; do you need one at each end of a double-ended train?

yup, you need a module in both ends for it to work :)

hmm. can we get a FAQ entry for that, perhaps?