Turn into Function Junction off the Sea to Sky Highway (about 8 km south of Whistler Village) and follow Alpha Lake Rd. Follow the road until it turns sharply to the right and turn into the parking lot for the Olives Community Market on the left hand side. Drive to the back of the building and park along the tree line opposite the building, this is where the Flank Trail head begins. We did this hike in the winter so there were some snow banks blocking the trail entrance, but there are signs so just be on the lookout!
Once you enter the trail there are handy signs that simply state "Train Wreck" with an arrow pointing you in the right direction. The trail can be a little tricky to follow but if you look out for signs and follow along the path you should be good!
Once on the trail you'll be led along the creek with some businesses and lots on the other side until you come out into a small clearing, keep to your left until the trail brings you to beneath the highway, cross under the bridge (above) and continue along as the trail should be easier to follow from this point on.
Tip: you need to follow this path on your way back to the parking lot as crossing under the overpass is the only spot to cross back over the river. We somehow missed the overpass on the way back and had to do some back tracking.
Be sure to follow the trail and not the tracks! The tracks are active so it's best to only cross the tracks when necessary and not follow them directly to the train wreck. We got lost on our way back and couldn't find our old footprints through the snowfall so we resorted to using the tracks until we could find a trail opening - the entire walk was spent looking back every few minutes to make sure there wasn't a train coming.