As far as I understood, the engine of the train was so to say an "eternal engine". But what was it really running on? Momentum couldn't have let it run for 17 years.
The Snowpiercer engine is describe as a Perpetual Motion Machine, in which case, yes, Momentum would let it run for 17 years.
Perpetual motion is motion that continues indefinitely without any external source of energy. This is impossible to ever achieve because of friction and other sources of energy loss. A perpetual motion machine is a hypothetical machine that can do work indefinitely without an energy source. This kind of machine is impossible, as it would violate the first or second law of thermodynamics.
This places it in squarely in the sci-fi department, as it's physically impossible, as we know physics anyway. As the Engine is slowly breaking and needs spare parts, that ultimately means it is suffering from friction or work losses. The spare parts are adding energy to the Engine, even if minor amounts. The Engine is only a mostly Perpetual Motion Machine.
Perhaps the train uses magnetic energy, not built in the track but in the earth. Say the track was laid following a global path of hop-scotch from one strong magnetic region to the next. Once the train gets going with a start from stored energy in a battery or other fuel like coal or even gas then it only needs to reach with [X] distance of the next magnetic region to get pulled up to it. All along the way, while being pulled magnetically, it stores kinetic energy from the spinning wheels to push past the region and on toward the next. Also the magnetic force can be reversed to push against the region behind it to aid with the necessary force.