Thank you - that is very helpful information. Both the 378 and 313s are in Pak128.Britain already, although I do not know whether the capacities are correct. The information on the overcrowded capacity is very useful indeed for Pak128.Britain-Experimental, which uses that feature. Would you have any interest in doing the research for the overcrowded capacity of all the vehicles in Pak128.Britain, and altering the .dat files accordingly? It would greatly expedite the release of Pak128.Britain-Experimental if you could. If you have to make educated guesses in some cases, don't worry, as any mistakes can be corrected in later releases.
Incidentally, did you mean that the 313s are less comfortable than the 378s, or the other way around? Funnily enough, I saw a 378 for the first time to-day at Stratford, but did not ride on it, as I was passing in another train. They looked quite good, though.
To answer your question, dual voltage trains can and will be accommodated using the way constraints feature by the simple expedient of not giving dual voltage vehicles any way constraint at all (but, being electric, they will still need some sort of electrification to run). I already have this set up in the sources for Pak128.Britain-Experimental.