Trainz Mac 2 should be able to operate on 4GB of ram, perhaps not as well. What matters more is your GPU (Graphics card) and CPU (Processor) - especially the latter, as Trainz taxes the CPU more than RAM or GPU.
Long Story Short: Yes, but it will be a bit laggy and annoying.
Long version: It should work fine but it can get a bit slow and laggy when you're running a big route, or anything with a lot of ploys. Also, if your computer doesn't have a lot of memory you'll run out once you install a lot of content . If you have a laptop, don't leave Trainz running and let your computer sleep or your computer will crash briefly and, another word of warning, save a lot cuz Trainz can crash.