For each train lapse rig you need one of these kits/parts:
http://www.hobbyengineering.com/H2031.html (gearbox with DC motor, built to 5402:1)
http://www.hobbyengineering.com/H2062.html (pulley system, using the 11m and 30mm pulley)
Two of these per rig
http://www.theoringstore.com/index.php?main_page=product_info&cPath=368_12_45&products_id=2041 (56mm Diameter 2.5mm o-rings.)
From radio shack
http://www.radioshack.com/product/index.jsp?productId=2062252 (holder for 2 AA batteries for power)
http://www.radioshack.com/product/index.jsp?productId=2062299 (25ohm potentiometer for speed control)
http://www.radioshack.com/product/index.jsp?productId=2062537 (to control the direction)
If you canâ€™t get a second hand G-scale train set, you will need wheels
http://www.amazon.com/G-Metal-Wheel-Set-4/dp/B0006O43PI (only built with these, tricky)
Plastic is easier to modify (sometimes needed,) but metal on metal track will likely be the smoothest motion. I have both plastic and metal wheel setups, the plastic was easier to do the initial build. The plastic wheels I used came from a garage sale, haven't found them for separate sale.
G-scale track of course
It is held together with very sturdy double sided tape.
http://www.amazon.com/Scotch-Permanent-Outdoor-Mounting-4011-LONG/dp/B000BL5INM (or similar)
GoPro mounting hardware can be used. If you don't have a spare thumbscrew and nut, use a couple a metric 5 mm-0.8 Stainless Steel Metric Cap Nut, and M5-0.8 x 20 mm Zinc-Plated Steel Hex Bolts.
You will need a soldering iron and some wire for connecting the DC motor, switch and speed controller (25ohm variable resistor.)
Build steps (total build time 2-3 hours) :
1) Print the base.
2) snip off the pads on the four arms (I found these helped for reliable printing.)
3) carefully drill out the axle holes to match the diameter of your train's wheels (carefully, I ruined many bases at this stage.)
4) Attach the two/three finger GoPro mount; as doing this later is tricky. I like the shortest 90 degree twist mount (included with a lot of accessories like http://gopro.com/camera-mounts/side-mount.) Two mounts allow you to counterweight the camera for extra-low shots (see pic.)
5) Use the tape to firmly attach battery holder with the screw away from the center, and the edge aligned with base edge (on the side of the switch hole.) See pics.
6) Build the kit gear-box to 5402:1 ratio (I haven't tried the other ratios for time-lapse.). The provided shaft is too long, so cut that in half.
7) Attach two 11mm pulley wheels to the gear box drive shaft. See pic.
8) Attach the 30mm pulley wheels to the train wheels. This part his tricky on some wheel sets, the pulley needs to be a close as possible to the train wheel to have some overhang to mount the axis into the base. See pic.
9) Tape the gearbox to the top of the battery container, shaft centered on the GoPro mount.
10) Attach the two o-rings to front and back wheels (this thing is all-wheel-drive for good traction).
11) The switch is mounted in the arm with the suitable hole. There is just enough clearance The wiring is simple (although fiddly.) The DPDT switch is setup to reverse the polarity of the power delivered to the DC motor (part of the gearbox kit.) The 25ohm potentiometer is in series with the DC motor to act as a very simple speed controller.
12) The potentiometer is attached via tape to any of the external surfaces (it is rather big, not many good places to mount it.)
You are now ready to attach a GoPro and time-lapse. If you use a HERO3 with a frame, and heavier metal wheels it will balance fine with the GoPro on the edge, otherwise extend with GoPro mounting hardware so it is more centered over the gearbox, or counterweight the rig. For the track, this is where the real flexibility is for this design, you can use straight track for a classic linear slide, or select track with different curvatures to move the camera through a complex path. The most common curve track is a tight 30-degrees per foot, these are fine if you are wanting to move around a corner, but I prefer the wider curves for most shots. There are a lot of option when buying track.
update 12/31/12: posted a blog entry on how I recommend shooting time-lapses with the new HERO3 Black Edition cameras. http://cineform.blogspot.com/2012/12/rethinking-time-lapse.html