Been explained already but I wanted to have fun with this one

Superman (electric engine) and the in this universe the clearly faster flash ( combustion engine) are having a race by pushing an extremely heavy train along a track

Well for Superman the weight is not an issue he just has to make sure not to rip through the train while pushing so he just pushes on the back of train at maximum force that won't destroy and steadily accelerates.

Flash is there with the problem of he is faster but weight is a problem... But with enough speed he uses the kinetic energy of a kick to get the train moving, then another and another

With each kick the train moves faster and faster and more energy is converted into speed.

Flash train now moving faster than Superman's but took a lot longer to get there