The Warsaw - Berlin express?
Or Warsaw - Krakow maybe?
More like the latter. The current speed record is 293 km/h, but in regular service not even the fastest Pendolino trains exceed 200. Hopefully with time the infrastructure improves.
Perhaps "This train is the fastest in Poland" is also acceptable?
We really don't like changing the subject of the sentence. And you changed the subject from the dummy pronoun "This" to "This train".
Imagine a situation in which your interlocutor doesn't even know that what you're talking about is a train.