There are definitely use cases for battery-electric trains:

• We have these in Germany usually in areas with low traffic. E.g. if a train line is only serviced a couple of times a day, it’s more cost-effective to carry the batteries with you than to electrify the line.
• Another use case are train ferries. They are the reason why Germany also had Diesel-powered high-speed trains for a while.
• Another challenge in Europe is the lack of harmonization of power supplies of train lines between countries. In cross-border traffic, trains have to be adapted to work with different energy supplies. Battery-electric trains can add flexibility for these scenarios. E.g. Germany uses AC 15 kV 16.7Hz, the Netherlands DC 1.5 kV on low-speed and AC 25 kV 50Hz on high-speed lines. When a train goes from the Netherlands to Germany, it disconnects from the Dutch system and reconnects to the German system on the fly. For a moment in between, the train loses power. If the train lacks momentum or has to stop unexpectedly, the train is stranded and has to be pushed over the border by another train that is independent of the power supply.