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.