Electromagnetic catapults were tested in the 1940s

Were navies experimenting with electromagnetic catapults shortly after the Second World War?

Yes, electromagnetic concepts were tested in the 1940s alongside early steam systems, but steam was the technology ready for practical adoption.

Electromagnetic catapult experiments date to the same broad period as the first steam-catapult work. The important difference was maturity rather than novelty. Steam systems could be turned into usable shipboard equipment with the technology then available, while electromagnetic alternatives could not yet offer the same practical path. Their later appearance on carriers therefore represents the maturation of an older idea rather than its sudden invention. Adoption depends on what can be made reliable at a particular technological level.