Normal accumulator holds 5MJ and in/outputs 300KW.
How about accumulator wagon? And if it holds more than 25MJ, why the recipe is so cheap?

It's 100 accumulators worth of storage and throughput.

The initial version (0.16.0) had truer balance and each wagon acted as 10 accumulators, but after using it for a bit it became clear that it needed lots more capacity - for example at a mining outpost you'd need to send about 10 carriages of accumulator wagons for each 1 carriage of mined ore. The capacity and throughput were increased 10x to make them a little more practical (and slightly more competitive with nuclear steam in fluid wagons), but balance-wise, it seemed a little silly to have to drop a solar-field's worth of batteries into a couple-car-long train - so the recipe was left at 10.

The fact that you had to cheat that much illustrates how moronic this idea would be IRL, and how realistic is vanilla factorio to that regard. Even at x100, a locomotive holding solid fuel holds 3 times more energy than the accu wagon.

It's really great to have it has a mod, to explore other options, good work!