As I mentioned above, fusion output scales with reactor size and magnetic field strength, and five seconds is the limit of their copper coils. There's no way for JET to significantly change any of that, so I don't know why you'd expect large improvements from them.
After 1997, the only way to scale up was reactor size, and that started with ITER, the 20-story-tall reactor in France. That soaked up a lot of fusion money, has been slow to build, and it's still not running. But more recently REBCO hit the market, and the same scaling laws say a reactor smaller than JET using those should get substantial energy gain. Two projects are building such reactors, and at least one will be ready around 2025.
(In any case, I wouldn't say five diesel locomotives are comparable to "burning a few tiny lumps of coal.")
