Hi C&TS488fan,
Joining NARCOA, passing its exams and having a NARCOA compliant motorcar allows you to sign up for an organized excursion through the NARCOA excursion coordinator(EC) putting on the run. Individual NARCOA members are NOT allowed to directly contact a railroad and run on their rail.
I am the NARCOA EC for the D&S and C&TS runs. There are a lot of mechanical requirements that must be met for these runs in addition to the NARCOA mechanical standards, and be aware that non-standard designs are definitely discouraged due to problems that have been caused in the past with reliability, compatibility and ability to stay on the rails. ECs have the discretion to disallow non-standard designs for this reason, and the NG railroads want to see traditional design motorcars on their lines in keeping with the historic nature of their operations. Also, the Narrow Gauge runs are demanding events due to traffic, grades and remote territory, so only experienced operators with proven designs are allowed (said experience gained at less demanding standard gauge events using SG equipment). We strongly encourage operators to gain significant experience with factory designed motorcars and with how NARCOA operates before undertaking the design and build of a motorcar of any gauge. Many NARCOA members are happy to let a prospective operator ride along on an excursion to help them understand the equipment and the way we operate.
But don't make designing a motorcar your first step. That is a good way to spend a lot of money on a yard ornament.
If you send me a private message with your email, I'd be happy to discuss this with you further.
Best regards,
Philip