The fast lines between Luton and St Pancras are full - or at least they will be when the extra East Midlands Trains long distance service is added. Its not simple for trains to make an extra stop - while they are standing they block the line, and headways (time between trains) are already at the minimum - with the line as busy as it is stopping one train just slows down the following one, and the following one, and the following one... And as Giles Davies rightly says, in the morning there is no room on EMT trains heading south to London, not is there any space on northbound services in the afternoon. The notion of adding time to long distance services, in order to save less than ten minutes on a trip between London and Luton is crazy, especially when the suburban service is (a) more frequent and (b) has more capacity. If Luton Airport Parkway needs more rail capacity, maybe a more viable solution is to make some of the Thamelink services non-stop on the slows, with some of them cutting out startions to the south.