Branches For Two-Way Traffic (Update)

If it can't really provide broad and fine-grained service throughout a metropolitan area, a mass-transit system is far from ideal. Buses can do that but they are so slow that anyone who can afford a car avoids them like the plague. Subways/Metros are faster and more popular but, because of their enormous cost, they are not fine-grained and never will be. Streetcars did it well a century ago, but autos drove them off the streets.

How did streetcars do it? In addition to their relatively low cost, they provided two-way traffic on the same street -- and, like any network, they had branches. Here is a map of the trolley system that organized and populated the Los Angeles region before cars took over.Some of it was single-track, but the busier portions were two-track, and there were more branches radiating out from the several hubs than you can count.

But providing branches for two-way traffic poses a tremendous challenge. Freeways face the problem too; they solve it with an overpass, which gobbles up a lot of acreage as well as money.

Now let's examine the classical problem for branches in two-way transit lines.


Leaving A on the trunk line, trains can go either way at the diverge switch. One setting leads to B, the other leads to C.

Returning from B or C, lines must merge at switch Y. Here, time-sharing (with an occasional stop) is inevitable. As at freeway merges, there is no alternative.
Look closer at X, where two paths cross. Railroads install small, inert metal "frogs" here, with no moving parts. Monorails could also cross here, but their geometry would demand a controllable pivoting piece. In either technology, trains from B could collide with those headed to C. Sometimes one train must stop at X and wait while the other passes through.

Not so for Rapid Transit, e.g. Metros and subways. At X the two paths are grade-separated, as at a freeway overpass, so collisions are impossible and no one ever has to wait at X. But the cost is awesome, especially because it's all under-ground. Uniquely, Project 21's branching feature boasts similar grade separation but, as seen in the video, with astonishingly low overhead bulk. Believe it or not, both diverge and merge switches fit onto the same factory-built module. Plus, the staggering cost of underground construction is completely avoided.

In all the world, no other two-way aerial branch is half this compact or simple. To my knowledge, the closest existing approach is the monorail in Jacksonville, Florida. They didn't try for grade separation at X but installed a movable frog instead. These two photos show it in its two modes.
With that bulk and complexity (and cost), you can see that such an arrangement wouldn't be acceptable for a network.

This is the main reason why Project 21 functions like Rapid Transit and constitutes a world-class breakthrough, compact enough and inexpensive enough to allow widespread networks of two-way traffic above city streets for the first time in ages.