It's not a bug, but I suspect it would break some older saves if it was done.
The three sets each, total six, of 4-wheel and 6-wheel carriages are basically the same thing with different liveries, so maybe they can be rolled into one?
The specs are all slightly different between Midland, LNWR and GNR, which I suspect was done because of the available engines at the time. I would suggest some new average for them.
4-wheel
intro: 1866
retire: 1911?
speed: 120km/h?
6-wheel
intro: 1887
retire: 1912?
speed: 135km/h?
This would probably be a sensible solution, and compatibility issues can easily be resolved using the compat.tab.
I am currently working on these carriages, as detailed in this (http://forum.simutrans.com/index.php?topic=8902) thread. Some differences will become apparent between the different companies' vehicles (the GNR, for example, having longer lavatory corridor 6 wheelers, the Midland having shorter non-corridor lavatory 6-wheelers, where only some of the people in the carriage could access the lavatory). There may be some scope for finding vehicles so similar to each other (perhaps LNWR and Midland 6-wheelers - I shall have to research the LNWR types more thoroughly) that they can be made into one vehicle with different liveries (in Experimental, at least).
ah, that is news to me.
In that case, it seems like they should be kept separate