It happens level change after boarding the new tram.
Screenshot: https://steamcommunity.com/sharedfiles/filedetails/?id=96123123
Crash Log: https://dl.dropbox.com/u/46003529/hl2_4104_crash_2012_9_14T22_17_45C0.zip
On the 2nd try it worked, but the map was laggy till the tram crashed.