Small engines are very different than big. They have very different needs re weight and complexity. A locomotive drive engine is basically mated to its power

Small engines are very different than big. They have very different needs re weight and complexity. A locomotive drive engine is basically mated to its power generating engine, moving in lockstep. Everything gets more complicated when one also wants to charge a battery. (Fyi, many locomotives have huge banks of resisters specifically for dumping energy into heat, which is not an option for cars.) Separating charge and power deliver requires two motors, a power-harvesting generator connected to the combustion engine and a separate drive motor. Those would need to be electrically separated since they would be always turning at different rates. That means totally separate charge and discharge circuitry, possibly even separate battery banks. Harvesting from the wheels might even require a third circuit to simultaneously charge from two sources. And the drive motor equipment would have to be sized large enough to cover the entire rated power output. Secondly, such a battery-in-line arrangement may create a battery-essential system whereby if the battery failed/discharged/overheated, the vehicle would just stop despite still having a working combustion engines. (The power direct from the generator may not be compatible with the drive motor.) Modern hybrids bypass all these problems by routing the IC engine to the wheels. The electrical system is then a single motor and an additional sprocket in the transmission. Power is then added or removed through a single motor and associated circuitry with no need to generate and provide electrical power simultaneously (F1 cars used to work like this). Add an extra differential, aka a "hybrid transmission", and you can run either motor alone or together (as in the Prius).