The speed of the trains/trucks is proportional to the length of the track/road. There is a fixed amount of time they get to run from the start to the finish; shorter run means slower speed, longer run means faster speed. They respawn every [I believe] 10 minutes, so they have 10 minutes to complete each run. You cannot adjust how often they spawn, only how fast they travel (based on road length) when you build a map.
