I was recently reminded about the importance of good, clean track. I pulled out an old micro layout, one of the first I built, just to run some trains back and forth ... oops. They just kept stalling, if they ran at all. So I pulled out the trusty Bright Boy and cleaned the track thoroughly. Now they ran OK except for one spot on the layout where there was a small problem with two track ends meeting. I had this problem when I built the layout, and had filed the track down to sort-of-fix the problem at the time, but had conveniently forgotten about this since that time ... I guess cognitive dissonance took over.
Newbies note, don't make my mistake: it is vitally important that you thoroughly test trains on your track before you start scenery. The time to fix problems, or to rip out sections and start over, is before glue and dirt and foam and paint are cemented down over everything. If the trains don't run optimally at this stage, adding scenery can only make it worse.