If time is relative (effected by gravity, or speeds approaching light) then wouldn't that create a paradox?

I know that time is effected by gravity or speed (when approaching the speed of light) but that really got me thinking about how that would effect velocity. say for example, that a train, with a m of (6.4 x 10^5kg) and this train is moving at the speed of light (299,792,458m/s) thereby slowing time for anyone inside. If you watch this train from OUTSIDE, it has a momentum of about 1.92 x 10^14 Newtons. However, if you are ON this train, time would slow down. So you would travel a greater distance in the same amount of time as an outside observer would see it as? Would you then have greater momentum then it appears to someone outside of this train? Or would you have less momentum than it appears to someone inside the train?