Railroad goof

Hi everybody, please tell me if my reasoning is correct or if I've missed something...

- Spirit gets captured, put on the train and dejectedly, in the company of other Indian horses, is transported to the soldier fort. The train ride supposedly takes quite a while.
- They are later made to work at the building of the railroad tracks, which as we know were in the process of crossing the continent from East to West. Spirit realizes this when he sees the sun setting in the direction towards which the tracks are being built. (west)
- he then starts havoc to avoid the construction of the railroad which will further the conquers of the white men (we know he didn't succeed in the long term, but anyhow).

Thus:
- if the railroad towards the West is still being built, how come did the Indian ponies were brought in from the West on a train???

I don't suppose:

a) the Lakota and the mustangs lived eastwards from the soldier camp;
nor
b) there was another parallel older railroad track to the new one being built...

Your thoughts are appreciated... ;)