The first thing is to check which version of tsection.dat file is installed. Route often install their contemporary version; the NWE route may have installed one which isn't compatible with one or more of your other routes.
This file is found in the Train Simulator/GLOBAL folder; open it with NotePad and check for a line near the top : _INFO ( Build 000nn )
where nn is a number. If the number is lower than 38, you need in install either version 38 or 42UK, both of which are available on this site (it doesn't matter which you pick) :
| ||Global TSection Issue 38 [310842 bytes] - tsection38.zip|
File ID: 27747 Date: 30 Nov 2011 - 256 Downloads
| ||MSTS Standardized global tsection.dat file Build 00042UK [325489 bytes] - Tsection42UK.zip|
File ID: 13758 Date: 05 Jan 2012 - 27149 Downloads
If you've already got one of these versions installed and MSTS still won't start, check the ReadMe files for NWE and Metrolink and make sure you've installed all the specified ancilliary files (like NewRoads, UKFS, Xtracks, etc).
If you still don't have any success, please come back
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