It is no small source of acrimony in the University of Cambridge that Manchester beat them to the first stored-program computer by one month with the Baby. However, once the Baby had been demonstrated, it was promptly dismantled, leaving Cambridge's EDSAC with the title of the world's first practical stored-program computer. The EDSAC was then used for nine years at the Mathematical Laboratory before being shut down in favour of the EDSAC 2.

Back when I was doing my degree, the 50th anniversary of the EDSAC came around, and we were given a project of creating a simulator for the machine. This we did, even including the rather interesting sound that the original machine made from a loudspeaker connected across the main mercury delay line. Yes, RAM was implemented as a long tube of mercury with sound pulses travelling down it and fed back into the beginning again.

The Whirlwind Project seriously considered using a microwave "delay line", a round trip microwave connection a long distance across the state of Massachusetts. Ended up using Williams tubes (https://en.wikipedia.org/wiki/Williams_tube) as did the Mark 1 and a number of other early computers (their random access had an advantage over delay lines), and eventually invented 3D magnetic core memory.

It has also been proposed to use a laser and one of the reflectors on the moon. That would give you a 300,000 km storage line. That proposal wasn't serious, though, if only because it is a challenge to keep that reflector in your field of view.

Another impractical delay line, but with variable delay, is the internet. You can hide some bits in a DNS request or in a Google search.