[masswerk]

The network is an interconnected system relying on average speeds and throughput. Slowing down or adding halts for the safety of a particular service probably wasn't an option. Hence the end of service. (The speed of that particular service consequentially dropped back to stationary infrastructure and transit between those hubs, roughly what it had been before 1838.)

[mulmen]

I think I’m getting it now. When high speed passenger service ran at 50mph it could also carry mail and deliver 8 times a day. When passenger service increased to 75mph it was too dangerous to carry mail the same way so mail service dropped back to once a day, probably on dedicated or slower trains.