I am actually new to this Rail Simulator, but have been playing Train Simulators since MSTS in 2002. I am looking to learn more about how to create new scenarios and I had a question about the San Diego Commuter Rail F59PHI.
Welcome to the forums! Unfortunately I am not an expert at scenario building but I know a few people here are into that and could probably help you out. There are also plenty of YouTube tutorials I'm sure.
Welcome aboard mdrinkle. Good to have you here. Just as PlatChap said, Youtube videos are a good source of information. For example, Matt Peddlesden has a Youtube channel called "Train Sim Live", where he has a number of videos on Train Sim scenario building. The videos focus on various different aspects ranging from laying tracks to signalling and landscape building. In addition to explaining the mere basics, Matt also gives good tips and some advanced advice.