Tramway are regenerating power and injecting them back in the grid when they brake. I think electric trains do it as well, the resistor grid is only used when not enough power in generated

Most electric trains only have a small service-and-emergency battery and don't have significant energy storage. For AC electric locomotives it's now standard to return electricity to the grid when using regenerative braking; that's harder with DC electric traction but is done in some circumstances (in others, the generated electricity is dumped into heat via a resistor bank).

The main barrier to electrification is the high upfront capital cost of the electrification works, set against the longer-term per-train return in lower fuel and locomotive/multiple unit costs. Lines which are not intensively used are difficult to justify electrifying on a financial-returns basis, and that probably includes most of the world's long-distance freight lines.

Nearly all fossil-fuel burning train engines (I think the exception being a handful of hydraulic engines in Germany?) are diesel-electric, which use the diesel engine to turn an electric generator to power electric traction motors. (Dynamic breaking is when those traction motors are used as generators, but that electricity is usually dissipated as heat immediatly.)