I don't think it's rude, and actually I didn't know there are trains with two tones for the doors, one for opening and one for closing. The trains I mostly use have a tone only for doors closing. And I think that's all that's needed in normal use. We know that when a train makes a scheduled stop at a station the doors will open, and a special sound before that happens is unnecessary.
As a more general comment, I think the colour red is very widely understood as meaning danger, watch out, something is about to happen. There isn't an equivalent sound with the same degree of widespread meaning. There are various series of fairly high-pitched beeps (this lorry is reversing, these doors are about to close, your microwave has now finished cooking), but not a standard one.