As yorkie has pointed out a flaw with what I was about to post (man he's got some brilliant mind-reading power!), I will first now explain how I came to use the name 'City Flyer', the line named so being most of the FGW network, mainly the Swansea/Cardiff Central to Paddington line:
At Royal Oak a while ago, on diversions that Sunday a HST left for Swansea at 1200 (M-F the Inverness HST also runs at this time, we'll get to that in a minute). It stormed out of Paddington as if there was no tomorrow. As it ran extremely quickly, I started calling it 'The 1200 City Flyer'. As the Inverness HST also runs at this time, this is the train the term, when used by me, refers to usually. It will also be used for the 0530 Swansea to Paddington, the reason for this being that it skips Swindon and DP as stops, running non-stop from BP to Reading!
Please note: This will no doubt cause confusion with the TL services with CityFlyer on them. Hence, I will always describe TL services not as CityFlyers, ALWAYS CityCrawlers. This may confuse you more, but when I am talking about, say, services from Bedford to Brighton, I will say something like 'The 1208 CityCrawler off Bedford was rather late by the time it reached West Hampstead'.
Also, you will come across heart-of-wessex and I using the term 'The Misery Line'. This refers to the Newport to Crewe section of the Marches route from Cardiff to Manchester. We named it so because we've both done it so much (I more so) it became really boring.
Another phrase either of us will use occasionally is The Timeless Line. This came about from having to do Hereford to Oxford, on the way to Paddington, on a 165. It took so very long I was falling asleep before Oxford. Time seemed to not exist on this line, that or it never moved, we felt a little (I did at least) as if we'd be on it forever.
I'm sure heart-of-wessex may have more to add.