No, sabotage doesn't hold up. The thing that rules it out is the braking, or rather lack of it. Passengers reported no screeching and no sudden deceleration before impact, which means the driver thought the line ahead was clear. Tampered track or an obstruction or some kind of remote hack (getting into the realms of fantasy there) would normally give him something to react to. This entirely fits a missed signal or a signalling/protection failure.
It also feels like "a train hitting a stationary one shouldn't happen", but that's the textbook rear-end-after-a-SPAD scenario - e.g. Ladbroke Grove. No official has hinted at foul play either. Everything so far points to an accident in the signalling chain or faulty TPWS. But the interim report is due in two days so we'll see more then.