You should also be aware that all vehicle traffic to/from Terminals 1,2,3 go through a long tunnel.
It takes about 20-30 seconds to drive through it.
I know this probably sounds funny but is it underground for a long time.
The Heathrow Express is only underground for a short time, obviously it has to go under the main runway, and then the M4 motorway, emerging at Stockley Park. From there on it links to the overground rail network. The entire journey to Paddington from Heathrow central is 15 minutes - so I guess it is only underground for less than 5 minutes
Heathrow Express trains are 'normal' size trains, so relatively spacious. (Though they do get crowded in peak periods). The platforms at Heathrow, though underground, are reasonably high ceiling - they don't feel claustrophobic in the way that some of London's tube stations do.
If that still puts you off - Cityjet fly from Dublin to London City. The DLR service from there is overground, and if you make your final destination on the DLR Tower Gateway you will stay overground.
If you fly BA and travel to Terminal 5, the underground section is a little longer and includes a stop of a couple of minutes at the (underground) T1/2/3 station.
As others have said, the trains and stations feel very spacious, but the route is very much underground for the last few minutes, longer for T5.
Thank you all so much for the info.It may seem silly but it can be a real problem.
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