Originally Posted by layz

I spend most of my time on the British Airways forum so it's easy to forget not everybody knows Heathrow.
I'll try and help you picture it:
Heathrow has three train stations.
Terminals 2&3 are in the central area, as was the now closed T1
Terminal 4 is then on one spur on the rail network
Terminal 5 is on another.
The Heathrow Express (the expensive train from Paddington) stops at T2&3 station first and then goes to T5.
TfL Rail (a cheaper option, formerly called the Heathrow Connect) starts at Paddington, stops at stations on the way, then T2&3 and then T4.
The trains are free between the Heathrow terminals but you pay to go to Paddington or any other destination.

The HGI is on the tube (London Underground) Piccadilly Line, the station is Hatton Cross and is one stop to T2&3 and two stops to T5, there's also tubes that go to T4 so check the destination screens before boarding. You can also use the tube to get into Central London, it's the cheapest but slower option.