24" is plenty wide for single-truck or even double-truck trolleys. This is a 24"x36" piece of MDF with a few pieces of Atlas flextrack (and a couple straight snap-tracks) with 7" radius curves. Not only does it run single-truck trolleys, I also ran my double-truck streetcars on it, which worked just fine (a Bachmann Brill, which you can find at train shows for under $20), but I also ran 50' Suydam double-truck interurbans on it with no problem at all (not connected into a train, of course, but you don't have to!) I also tested my Bachmann 44 tonner, which ran just fine on these curves.
The lower right corner is a little tricky (I measured it out and it's actually a 6" curve not 7") but I can fix that with a few minutes and a razor saw. My plan is to turn this into a 2'x3' mini display streetcar layout, with a block of houses in the middle--a very common streetcar "pizza" layout plan from the old days of trolley modeling. If you wanted a bit more safety, you could build something like this and scoot the tracks back an inch or so from the edge, and expand the curves to 8-9" radius, but I like to live a little dangerously.
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Also, this is just an experiment I whipped up in an evening to spur my plans for a mini layout--my main layout is an around-the-room interurban shelf layout, which has mostly been point-to-point but I recently completed a project to make an entire lap around the room (disrupted by a plan to flip the entire layout so I don't have to duck to enter it.) It's a lot more generous in terms of space constraints, since I'm running multi-car trains and pulling railroad-standard (for the 1940s) freight cars--12" minimum radius, 15" on the mainline, running both passengers and freight. And streetcars.
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As far as your plan (shelf layout with dog-bone ends) I'd recommend strongly considering making the middle of the "dog bone" even narrower, and just model one street with two-way streetcar traffic; you can push the tracks toward the front of the shelf and model full buildings, or model them as background flats, which would be a pretty satisfactory approach. My own layout is mostly 12" deep, but I do have a couple of stub peninsulas, one 3' deep and 2' wide for an industrial spur, and one 2' deep and 4' wide for a city scene--and it's where I have the most trouble reaching in to clean and maintain track, despite the farthest track being only 21" from the layout edge, because of the overhead messenger wire (I don't model live overhead) and the tall buildings in the foreground! However, it's manageable, and my new plan will let me access it from both sides; all the rest of the sections I have planned will be 12" wide or accessible from both sides! And if you're planning this as a table rather than a shelf, you could model both sides of a single block with single or double track streetcars in 24-30" quite comfortably.
You could also integrate a streetcar scene into a future model railroad (I'm a big advocate of growing sectional shelf layouts from a single module)--you could have a streetcar loop that runs to the front steps of a railroad passenger train station, then build the passenger railroad from the station to its destination when you have enough room!
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Sadly, trolleyville.com has gone dormant, but you can find a lot of incredibly useful information on the archive.org copy of Trolleyville on streetcar modeling.
