I haven't used Sketchup so I don't know what functions it has, but you could try Blender (which is what Guy and I use). The coils of the spring are spirals/helixes and relatively easy to do. The bogies look complex, but like everything in 3D modelling it is just a case of building up details from simple shapes.
I am not sure where you live (with a user name like your's possibly the Eastern edges of Cumbria!), but if you can get to our show on Sat or Sun Guy will be demoing Blender design (2-4pm each day): http://www.demu.org.uk/showcase.php
Cheers, Mike