For long distance services there are no extra trains, crews, paths or the ability to arrange things at short notice. Virgin operate roughly the same service throughout the day, there aren't many trains in sidings between the peaks. Running a London Midland train to Manchester isn't straight forward - the trains are slower than pendolinos, there will be problems with route & stock knowledge, etc. For shorter distance service thee is no reason why you can't run longer trains - Chiltern could strengthen services to Birmingham between the peaks (but not during the peak). Also remember that long distance services are the same as airlines in that load factors have increased with some very cheap fares available (if you book far enough in advance at a quiet time) and there are fewer spare seats. Railways are unfortunately less flexible than airlines.