This is one of the longest rides I've done in this area. It's an enjoyable day-long ride which takes you through rural northern Allegheny County on the Red Belt itself, and also follows along the Allegheny and Ohio River valleys on the outbound and return trip. Most of the Red Belt is surprisingly flat; there are a couple of fairly short hills, but the rest is just smooth or rolling countryside. All the same, if you're new to Pittsburgh biking, I suggest you try shorter rides along the river valleys first, before working up to this one.
Start out by heading up the Allegheny River toward Tarentum. My favorite way to do this is to take the Allegheny River Boulevard out of Pittsburgh, cross the river at Oakmont (on the Yellow Belt), and then turn right and head up Freeport Road, which used to be Route 28 before they built the highway. You'll have the river in sight for much of the way, and you'll pass through towns like Verona, Harmarville, Cheswick, Springdale, and finally reach Tarentum itself.
When you reach Tarentum, you'll see signs for the Red Belt going off to the left at the point where a bridge over the Allegheny goes off to the right. Turn left onto the Red Belt. If you're doing the full loop, you can then just follow the Red Belt signs for its entire length.
If you're doing only some of the belt, or otherwise have time to spare, you may find the Tour-Ed mine an interesting stop. On weekends until about 3 they give guided tours of a working coal mine. (The mine has stopped extracting coal, but I believe they will still continue to give tours.) The entrance is a sharp right turn uphill (nearly a 180) off the Red Belt a few hundred yards past route 28.
A few miles in, the Red Belt jogs right to cross a railroad bridge. If you're biking a shorter loop, you might want to cross the bridge and then turn left on Saxonburg (a popular bicycling road in its own right), and follow it all the way back into Pittsburgh. Or you can try the flatter Little Deer Creek road, on the near side of the bridge, which runs back to Harmarville. But for the full-length ride, continue straight on.
There's a small village (Bakerstown) where the Red Belt crosses under route 8. This is essentially your half-way point, and I recommend stopping here for food and drink if you haven't done so already. I don't remember any sizable towns on the Red Belt for a while after this. There is, however, a store which sells apples and cider somewhere on the second half of the Red Belt, amid the many old farm estates in the Mars area. That's well worth a stop too, whether or not you brought your own food and drink.
On the last segment of the Red Belt, you'll follow a brook as the road straddles the Allegheny-Beaver county border. Eventually, you'll come out onto the Ohio River.
Technically, the Red Belt ends at Ohio River Boulevard (Route 65). You could turn here and follow it all the way back into Pittsburgh, but it's not a particularly enjoyable road for biking. Instead, just _before_ the Red Belt ends, turn left onto Beaver Rd., a less-trafficked (and more scenic) road which runs parallel to route 65 for several miles. (At the outset, Beaver Rd. will be very close to route 65, so it's hard to miss.) This road is a narrower, tree-lined lane which takes you through residential areas, private school campuses, and the shopping district of Sewickley. Once in Sewickley, you can continue straight on Beaver Rd. if you like, but that will eventually dump you out onto Route 65. A preferable way to head back to town is to turn right at the Orange Belt, cross the river, and then take the "ohio-valley" route in reverse. (See the "ohio-valley" ride file for the routing.) Or, turn left at the Orange Belt, follow it to Rochester Rd., and then turn right to take that back to town. (See the "rochester" ride file for that routing.)
John Ockerbloom (firstname.lastname@example.org)