You can, if you can find it. With some Vista installations it's in Virtual Store which can make life difficult.
In addition, the Trainz screenshots folder stores the images as .tga files which won't load into MS Paint.
You therefore need to use another application which can read .tga files. I use Gimp2, which is great for gmax image processing etc.
However, it is unnecessarily complex to use if all you want to do is grab screenshots and save them as .jpg files and maybe change the sizing for forum posting. I prefer the simplicity of MS Paint to do that.
I do have to remember to delete my Trainz .tga files at regular intervals though. I've simply created a desktop shortcut to the Trainz screenshot folder to make life easier than having to remember where they are stored and groping my way through two file pathways.
I hate Vista!