I have several Bachmann trollies that have run briefly when I get them and then do not run at all.  I have tried the routine wheel cleaning, contact cleaning, and lubrication, but nothing more involved.  Is there a common problem ( contact with the rails; a frail wire; etc.) that I need to address?  I would like to get all of them running in a manner that they stay running and are not a problem.  I am using a good quality power pack and everything else works, including other Bachmann locomotives.