MARC train simmers: I am close to releasing my MARC PennLine activity pack, which includes all 44 PennLine trains completely player drivable and based exactly upon last year's timetable. Default equipment is NALW's NJT ALP44, but a MARC AEM-7 is slated for release soon by the same author. Passenger cars are MNRR Horizon coaches, the best I could do for lack of MARC painted ones. No Amtrak traffic as of yet, that will be done after I make my Amtrak NEC pack (all 82 Amtrak NEC trains!). I also authored a MARC Camden Line activity pack last Summer. Version 2 of this pack will be released soon, pending the release of a MARC GP40WH-2. It will include a fixed southbound path and push/pull control. All activities run on the NECv4. Grab all of that stuff on http://www.train-sim.com (everything is freeware). Scroll down to the bottom of the page and click on the "File Library" button. Register for free if you don't have an account and log in. So far only Camden line version 1 is there, but PennLine will be released early this week. I'll let you know when it is. So there you have it: 2 out of the 3 MARC lines completely player-drivable in MSTS!
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MARC in Microsoft Train Simulator
- Discussion related to DC area passenger rail services from Northern Virginia to Baltimore, MD. Includes Light Rail and Baltimore Subway.
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Yes, that is NECv4. It does Septa and MARC on the NEC, as well as the parrallel CSX/MARC Camden line, light rail lines and Washington Metro. It's amazing. My activities run on NECv4, so you'll need to install it before adding in my acts.
If you make a Brunswick Line route, please let me know. maybe by then I will have a computer that actually will run my copy of MSTS. 
Unfourtunately, you are asking the wrong guy to make routes. I just do the activities. The release of the PennLine pack may be delayed while I wait for a new equipment pack to come out, which will contain some really detailed MARC equipment.
Don't hold your breath with MSTS! I was waiting for a Hudson River Valley Line for years, it never came out publicly!