No, it wouldn't. Multiple sized tracks were used all the time.
What you're proposing is scaling down the trains to fit on the narrow-gauge track. Narrow gauge was used very often in mines and that because they were cheaper to build and were able to have tighter curves.
The larger gauge, the ones you see on the big trains in TF2 was (and is) used over large distances with large cargo because they could achieve higher speeds and hold more weight (Allowing for up to kilometer long trains)
As you can see, there's plenty reason for narrow and standard gauge railways to be together, perhaps the international railway passes by your humble mining town which uses narrow gauge because of it's inexpensiveness to build. This was often done in real life when the mine/logging complex used narrow gauge and then transferred it to the standard gauge trains nearby in order to cart it off to distant cities or countries. (This could also make an interesting building to play in!)
Larger gauged trains can run on smaller gauged track (although the length of the engine/carts in TF2 wouldn't take the tight curves very well at all) but they don't do it by scaling down, they do it by exchanging bogies (the little things that their wheels are on).
If you were to scale the bogies down to fit the narrow gauge track, you'd see how absolutely ridiculous it would look and how inefficient they'd take to the track curves and slopes.
There is already a narrow-gauge steam locomotive in the game, I recommend you use that.