4g uses high speed LTE technology on standard cell towers using standard frequencies in the 700-2300mhz band.

5g uses frequencies in the 25-60ghz band.

This also means that the range of each tower is much, much less. It is small cell technology that requires 20 cell towers or lamp posts to provide service where one 4g tower could do the job. But the speed offered in these small cells is much faster than 4g, and frequency re-use becomes so much easier.

But

From my understanding, chorus wont offer a UFB connection to a public roadside which could be a problem. To order a UFB or fiber tail, it needs to be delivered within a private property boundary. All those lamp posts with picocells on them require a fiber feed. Unless they have some fancy deal.

Trains

For trains in tunnels they can use leaky coax. But yes they do pose a problem where the distances that each pico cell covers wont allow a cellphone to lock on for very long and much airtime will be wasted in handovers between the picocells.

I expect fast moving devices to remain on the existing 4g towers.