My train that was travelling on a high speed rail section at low speed passed over some standard rail pieces. I then destroyed the standard rail pieces to replace them with high speed, I noticed that the dropped rails were high speed ones, I'm pretty sure this is a bug. I will try to experiment tomorrow and get a repeatable setup for more information...

Ok, I managed to get this to happen again last night; it seems to be that when a train explodes due to going too fast on a transition without a transition rail, IF some of the train manages to keep going, any tracks it passes over APPEAR to change to standard rails, ie just drawn wrong. But when you dig them up they revert back to HS rails. Not sure if this makes sense? It only seems to happen when I mess up placement of HS transition rails though lol.

Sounds like net sync issues.

I can confirm having seen this, but it only happens when in creative and you rapidly replace one track with another. A block update nearby fixes it.