The only drawing programme we have found to our knowledge that runs on Macs.
Quote from their website
RailModeller Pro is a planning tool for designing model railroad and slot car layouts.
Key features include:
- Layouts of up to 10 x 10km (6.2 x 6.2 miles) in size,
- More than 250 multi-brand libraries with 6000+ tracks and accessories
There is a free trial version available