This question is similar to one for PF motors.
The 12V motor bogie has considerably more resistance than does the newer PF bogie motor, which may be moved very easily when not operating.
So, if one pushes a 12V-motorized train by hand, will it cause damage to the motor?
See title "Set 7865: six wheeled Lego® train motor (12V, 1980 - 1990)" on this page for the specific motor I'm referring to.
Side note: I've noticed that the connected headlights in the train don't illuminate when pushed by hand (and have asked a question specifically in regards to this).