To be fair, I think the rail guns "Gustav" and "Dora" were reasonable developments for the specific task they were meant to fulfil -- smashing the Maginot Line.

In the actual war, though, the Maginot Line was bypassed, leaving these guns without a purpose.

The waste of effort was exacerbated by sending them many hundreds of miles to the Eastern Front, instead of a few dozen miles to the West as had been originally intended.

Notwithstanding the above, if the Germans had developed a heavy bomber/penetrating bomb combination like the Lancaster/Tallboy, it would have been a much better and more flexible weapon.