AtD translation: getting his backside to the trackside

[Mike Jing]

P179.10-16   What in Creation could be going on up here, Lew couldn’t
figure. He had a distinct, sleep-wrecking impression that he ought to just
be getting his backside to the trackside, head on down to Denver, and not
come up here again till it was all over. Whatever it was. It sure ‘s hell
looked like war, and that must be what was keeping him here, he calculated,
that possibility. Something like wanting to find out which side he was on
without all these doubts. . . .

What does "getting his backside to the trackside" mean exactly?