Trains are not officially supported to run at extreme high speeds, the official trains are all within the normal range but modders have the ability to push the limits beyond that

these speed issues may be fixed in the future, but it's not planned for any time soon

Originally posted by aivan.markus:

For some reason, engines will speed up so fast they will decouple. this happens to a couple train mods (mostly Sp 4449) and i don't know how to fix it. please help!!

Regarding the 4449, I have capped its maximum speed (at the default game speed setting) to approximately 110 miles per hour (accurate to the real locomotive's top speed), which shouldn't be fast enough to cause any decoupling issues. Is there a specific version of 4449 you're having problems with? I might have messed something up somewhere.