But something about travelling in crowded and closed space like train with a fully charged diy (or even commercial) battery pack sounds risky.
This is one of the reasons I didn't go for a diy replacement of TWS pair with dead batteries.
The welds on this pack are very good, I have checked and re-checked all of them, inspected the pack using a FLIR while running at maximum charge / discharge just to make sure there is no hidden jump in resistance or a bad cell. The whole thing is wrapped in shock absorbent foam but it still is a massive pack and if it were to go into thermal runaway there is just too much energy there to be dealt with.
It stays outside and is charged outside and I'm monitoring the cells groups to mV precision to make sure that they're not misbehaving. I'm not taking any risks with 170 charged 18650s, if I would so much as drop or bump the bike I would discharge the pack and dismantle it.
I know it was not your intent, but when people hear this stuff about the fragility of EVs, it really makes them feel safer with a gas motor and a tank of liquid that can take a few dents and whacks without completely exploding.