Collisions between motor vehicles and trains at railway level crossings have been a high-profile issue for many years in New Zealand and other countries. Errors made in judging a train's speed could possibly be attributed to motorists being unknowingly subjected to a size-speed illusion and this could put them at considerable risk. Leibowitz (1985) maintained that a large object seems to be moving slower than a small object travelling at the same speed. Support has been provided for Leibowitz's theory from studies using simple shapes on a screen. However, the reasons behind the size-speed illusion remain unknown and there is no experimental evidence that it applies to an approaching train situation. To investigate these issues, we tested observers' relative speed estimation performance for a train and a car approaching at a range of speeds and distances, in a simulated environment. The data show that participants significantly underestimated the speed of the train, compared to the car. A size-speed illusion seems to be operating in the case of the approaching train in our simulation and may therefore be a risk factor in some railway level crossing collisions.