Those 21 suggestions might indeed improve reliability by giving more flexiblity in short workings but GNE dropped the 21A,B etc etc because 'passengers found it confusing apparantly to have more than one 21 service
I can't see the point of a 21A and the X21 you propose, you'd be duplicating.
I take it you wouldn't replace the 28A? Leave the 29 to serve Kibblesworth on a weekday and the 24A to pick up the rest of the slack?
RE: Service Suggestions