TS12 is an example of backwards compatibility because you can run TS2009 and TS2010 content in it, but not TRS2004 or TRS2006 ONLY content. (I believe there is a compatibility mode option that you can turn on, but not everyone uses it when the piece of content is from TRS2006).
I've been around since the early day's and to the best of my knowledge, there's no magical switch that you can turn on or off for "compatible mode" The last version of Trainz that is viewed as having this "compatible mode" is TS2012
49922. After 49922, things tightened up on errors/warnings CHECKING in CMP. A lot of content that was showing "errors/warnings" in CMP for TRS2006, TS2009, TS2010 & TS2012 49922, yet still worked in those version, started showing as "faulty" in CMP for TS2012 57xxx or above. (TS2012 SP1) and a lot of it won't work in TS2012 57xxx or above as is.
After TS2012
49922, you could no longer install Murchison 2, S&C, Carbon City using the .ja file method. TS2012 57xxx & above is now all DLS (downloaded from N3V method).
I have 180+ (approx) 3ft (36") Steam Loco's, Diesel Loco's & Rolling Stock. At a guess, 90% works OK in TS2010. 75% work in TS2012 49922 and only about 35%-45% works in TS2012 57xxx, TS2102 SP1 HF4 & Tane. Note; Ninety six of this 3ft content still shows errors in TS2012 49922, yet 80% of that still works in TS2012 49922...
I'm hoping that someday most of the "faulty" 3ft (36") content will be repaired up to
Version 3.6, so that it will run error free in TS2012
49922 & above...
Many thanks to guys like
Bendorsey, Philskene &
(a few others) that have already either repaired some of the faulty 3ft loco's & rolling stock.
(They now work in TS2012 & Tane). Or they have created new 3ft gauge rolling stock that works in TS2012 & Tane.
(Cheers guys).
Here is an example of one of the 3ft rolling stock that has recently been repaired & is now on the DLS.
Cheers, Mac...