Or use custom content manager
Open the 'Manage Content' app in the autoplay menu. Find your loco (easier said than done) and right click 'Edit'. this will open a second menu, left click 'edit in content creator plus'. Hopefully this will bring up the loco assets all nicely itemised; scroll down and you should find "speedlimit". Change as desired, click 'file' and 'save', close the program. Return to Content Manager and right click the loco model, 'edit' and 'commit'. a dialogue will appear saying "committing changes" and hey presto it should all work.
some locos need to have their asociated "enginespec" dependency files also edited to give them more power to reach high speeds.
Thats how i always do that sorta tampering; trawling through config files is a pain.
