If you're referring to power in the strict physical sense, aircraft engines are typically more powerful (in Watts) compared to train engines.
For instance, one of the most powerful engines globally (the GE 90-115B, notably used in the B777-300) produces roughly 50 MW of power (around 5% of a nuclear reactor's output). By contrast, a highly powerful locomotive (like the GE AC6000CW) generates under 10 MW.
That said, when it comes to tractive (or propulsive) force delivered by the engine, things flip. Locomotive engines produce very strong tractive force at low speeds. For example, the locomotive mentioned above delivers roughly 85 tons of tractive force at low speeds, while the jet engine above (despite being more powerful) only produces 55 tons of propulsive force at low speeds.
However, the jet engine doesn't require support (rail) to generate this propulsive force, and it keeps producing this force even at extremely high speeds (whereas a locomotive's tractive force drops off sharply as speed increases).