I just remembered... I actually wrote a bit of flash fiction relating to this. The situation was that the NEC between WAS and WIL got so bad that you were better off with diesel service stopping at all MARC and SEPTA stops until you hit WIL and could offload to SEPTA (long-distance service would switch to electric in PHL).
I'll spare you the story (it's part of a collection that was written as part of No Novel November). Instead, let us indulge into a mental exercise. The goal is 20 minute headways WAS-WIL. The constraints:
- MUST USE MARC DIESEL EQUIPMENT AND CARS. Amtrak long-distance diesel service switches in PHL but falls under rules below.
- Must make all stops. Station stops are block boundaries, and your signals are track crew with radios.
- 79 MPH limit (Rule 556)
- Two track service. All other tracks are OOS. Switches are hand-set.
Looking at MARC's current Penn Line schedule, we can safely assume all stop service at current speeds (100 mph from my own measurements) is roughly two hours to Perryville. Wilmington is another 20 minutes away, with Newark splitting that. If we cut the speed down to 80 mph, it's very likely we get to Wilmington in about 3 hours.
20 minute service needs 3 train sets per hour in a single direction. 18 train sets total in push-pull. If you double-up the MP36's and use the SC-44's, that'll leave MARC with 12 engines for Camden and Brunswick line service (14 if you use the GP40's). If you give them 6 double-level car consists (MARC III and IV), it's going to leave 3 of those left for other service (on top of the 54 single-level MARC II's).
Brunswick line needs 9 consists for regular service and is rush service, rush direction (3 on restricted). Camden Line needs 11 for the day, but can turn some consists back (4 restricted). Those numbers are for a single direction.
Without doing some explicit scheduling of consists, everything looks to lean very possible... but 20 min NEC service is the limit.