How to curve a complex group along a path

You could try and make it into an art brush and then apply that brush to a curved path.

This will only work when there are no gradients in it.

Hillary,

You can:

1) Create a copy of the part of the track/path to carry the train, with a length that equals the length of the train (you may cut that part out and delete the rest from the copy);

2) Select all the locomotive parts and Group them, then Object>Transform>Reflect a copy (Horizontal axis) to fit underneath the track/original and set Stroke/Fill to None (or Opacity to 0), then Group both and turn them into an Art Brush;

3) Apply the Art Brush from 2) to the copy of the track/path from 1).

Edit: As Monika has rightly pointed out (the rifts in the time space continuum of the forum will soon make it look as if she is only about to say it), if you have gradients in the train design (I assumed it was simpler than that), you will need to do something differently; it may be possible to replace gradients with (smooth/high step number) blends before the first Grouping in 2).

have you tried Puppet Warp?

https://helpx.adobe.com/illustrator/using/puppet-warp.html