Yes it does joins and spilts. Coventry sometimes as well
As I’ve already said to @RealTrains07 go on real time trains. Search the times at a particular hour of an average mid day and it will show you there. There is One split and join at New st per hour. There are one off splits at Northampton but not regular ones. Most are through services there. There are terminators at MKC and Tring. You can tell by the start or finish of a service roughly at the same time as a through service on the same platform. There are your clues for splits and joins. The same head code can’t run to two different places one after another. They have to have different head codes.