1500V DC

anme said:

Is there ever discussion of converting 1500 or 3000V DC lines to 25kV AC (or similar)? I appreciate this would be a big undertaking, but could it be worthwhile if done, for example, when electrical infrastructure was renewed?

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Slovakia are apparently going to switch from 3kV to 25kV in 2023.

Poland (currently 3kV) have talked about switching the whole network, but it sounds like that's not going to happen. However they have apparently decided on 25kV for the new build "CPK" (higher speed rail), and they might also be switching one highish speed line to 25 kV (the CMK has 250 km/h geometry, but is run at 200 km/h for power supply reasons, and they're trying to make 230 km/h possible which will likely involve a switch). Most polish loco's are 3kV, but they do have some newer multi-system locos and trains (Pendolinos) - and those are probably sufficient to service that line. (I'm pretty sure all the older locos that are only 3kV compatible also happen to only go up to 160 km/h, which stops them from using that line anyway).

The EU requires 25kV on new high speed lines. I think exceptions are allowed when linking to existing systems but probably only with provision to upgrade to 25kV later.

For real high speed you need AC anyway in order to convey the power required. But I doubt very much that a 25 kV high speed line will ever be built in 15 kV territory.

I have been vacationing in Croatia last summer, and observed tha on the line from Zidani Most, the catenary had been converted from 3 to 25 KV all the way To Zagreb. I do not know where the junction between 3 and 25 kV used to be before, but old italian style catenary poles supporting a new 25 kV catenary can be seen all the way. So it seems that Croatia is now an homogeneous 25 kV territory.