IMO I think it's better to not make an assertion than to make an assertion that your not willing to back up with rationalist or empirical evidence
This itself is an assertion
They're allowed to make assertions and speculation for future events, because that's what they are supposed to do. Remember, this all based upon another assertion: that Labour are going to be in a position to act on such a budget, so they follow the what ifs, including what's a fairly sensible suggestion regarding rolling stock, which should be taken into consideration as to whether labour consider it a bad/expensive idea. I would much these ideas be tested now and not weeks or months after they get approved by the Exchequer.