I've looked through the page about luggage at their website but there's no mentioning of e-scooters. Does anyone know if they're allowed?
No, you can't bring an electric scooter on-board MTRX trains in Sweden.
From the general travel conditions:
10.4 Bicycles (including electric bicycles), electric scooters, electric mopeds and similar vehicles cannot be brought on-board.
However, they're currently allowed on other some other Swedish trains travelling the same routes.