There are some complex issues that have been raised. Firstly, length: it is generally preferable for all carriage graphics to have as accurate a length as possible. The lack of granularity in the length parameter is unfortunate, but all sorts of visual chaos will ensue if we start having carriages of deliberately the wrong length to work around this. Far better would be to work on allowing fractional lengths in the code (although this may be some way off unless someone else codes it).
Secondly, luggage: any representation of luggage in comfort must take account of the fact that the luggage carrying capacity of one vehicle in a train can affect the comfort of other vehicles. Doing this would require a change in the code (catering values are unsuitable, as they generate revenue as well as increasing comfort). For this reason, the simulation of luggage cars has generally been omitted, and, if included in some cases, would require revision of the entire pakset to add luggage vehicles to match all of the various railway carriages, which would be a very large task.
As for overcrowding, the policy so far has been to allow overcrowding in all but first class carriages/compartments. The general rule of thumb is, for non-corridor compartment stock, the overcrowded capacity is half as much again as the seated capacity, as generally as many people can stand in the middle of a compartment as can sit on either side of it. The same holds in corridor compartments, but people can also stand in the corridor, giving a higher ratio of overcrowded to seated capacity, and the same in saloon carriages.
Although people may prefer to stand in the corridor given the chance, people would undoubtedly have stood in compartments in corridor carriages if there were no other space available: it is not plausible that people would have refused to get on the train at all just because they have to stand in a compartment rather than in the corridor. People will stand anywhere with enough space if the train is crowded enough.
Edit: I have now had a chance to look over the code. I see that you have re-rendered quite a lot of vehicles: that is very helpful: thank you for that. I do take the point about the front/rear full brakes, as the graphics do show them having a particular end.
However, one thing that I do notice is that you have in some places (e.g. the LBSCR-4Wheel-First-fitted) added first class without modifying the capacity or comfort. The carriage, although modelled on a first class carriage, had the capacity and comfort that it would have had had it been built as a long-distance third class carriage: before classes were introduced, in the LBSCR carriages, class was used as a proxy for whether the carriages were long distance or suburban.
You will thus need to recalibrate the comfort and capacity of these carriages to match with them being first class (i.e. three a side in each compartment and with a comfort matching contemporaneous first class vehicles). For reference, this is what an LBSCR 4 wheel first class compartment looks like:
LBSCR 4 wheel first (interior) by
James Petts, on Flickr
LBSCR 4 wheel first (interior) by
James Petts, on Flickr