Beyond the technical aspects of designing an activity, which has been covered, I suggest you think about the activity 'story'. Having a story line always makes activities more interesting to the players - whether its getting a time freight over a crowded mainline, keeping a passenger train on schedule in the face of traffic, or just a simple switching activity that blocks cars for pickup.
Keep the first one simple. As you get more experienced, your activities can be more complicated.
Chris
"True rail fans have two favorite railroads. The B&O and one other."