I guess. And it would be a tad too
obvious unbelievable plot-changing distracting to have the Joker accidentally riding the train.
But... I actually seem to recall reading a novel called Pelham 123 back in my early teens (ie. some years ago!
) and I can't recall much if anything of the plot except this train gets hi-jacked by bad guys and stands there for quite a while.... which presumably means the passengers on the train have to stay around for several scenes. Which again makes it less likely that the cowboys are cameo homages to another film or actor...
Time will show.