I have successfully mated Gargraves flex track (and ross turnouts) to Atlas scake track used in their bridges, and to tubular track used by Lionel in their Bascule, Lift, and Swing bridges ... to do this, there are special pins which mate to each ... some come with the Atlas bridges, and I no longer recall where the tubular special pins may have originated. The following usually works ... just put the normal gargraves pins into ross turnouts, and the other end into tubular track, and wedge it up, so it is a fairly tight fit, you can slip round tootpicks into the tubular track first if desired ... I do not really know what MTH track might be like, but all of these tracks can be mated up if you are even slightly patient ... good luck and best regards, richard
Prototype railroads had lots of transitions, from one weight rail, to another; at lift and draw bridges, at turntables ... so this is normal ...