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.
So I’ve got to the end of series one and it looks like my magnetic drive idea is wrong. From the clues in the script I now suspect the Snowpiercer engine is a collection of nuclear isotopes that put out a constant but fixed heat. You’ll remember the one used in The Martian to warm the Mars rover. Though reliable they are highly radioactive if the containers are broken open there’ll be a whole world of trouble for the train. Sadly though this won’t explain why the train can’t stop.
I’m just watching series 1 an have a theory about what powers Snowpiercer. You’ll know from school physics that to make electricity you move a magnet through a coil of conductive wire, or in this case move a coil of wire through through a magnetic field. If the coil was long enough (say on a five mile long train) you could (theoretically) generate electricity by moving the coil through Earths magnetic field. The faster you go the more power you generate! This explains why Snowpiercer had all those empty carriages on the end (that all the Tailies were able to stow away in!) it just had to be that long to work!
If it runs on Hydrogen (as we saw last night) which could be produced by using electricity generated by the motion of the train axels. Maybe if they keep the train above a certain speed, it generates enough extra electricity to make enough hydrogen to power the train electrically. Proper routing of the tracks to utilize downhill stretches to generate extra power using the "Willford Module" to regulate the whole thing. Physics wise I don't think it would work but maybe the guy really is a genius. My real problem with the whole thing is what will they do when the train goes off the track or there is too much snow or an avalanche destroys the tracks, etc. They all die because no more perpetual motion.
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.