When it comes to minimum street/rail corridor widths, the Seattle light rail line on MLK provides excellent examples. Squeezing the corridor tighter will be unsafely uncomfortable for trains, vehicles, and pedestrians. The light rail line runs down the middle of the street.
MLK in Seattle generally has 10 feet wide traffic lanes.
100 feet for 4 traffic lanes, 2 tracks, and 2 side platforms
90 feet for 4 traffic lanes, 2 tracks and 1 island platform
70 feet for 4 traffic lanes and 2 tracks.
Metro running on Main Street in downtown Houston makes a better example for running dedicated lanes in tight quarters.
It's minimum width is 55 feet for 2 traffic lanes and 2 tracks in the middle of the street, with a small 10 feet wide landscaped median between the tracks .
It's also 55 feet wide for 2 traffic lanes, 2 tracks in the middle of the street, and just 1 side platform 10 feet wide at its maximum.
The traffic lanes in downtown Houston are also 10 feet wide.
If we dropped the 10 feet wide landscaped median, the corridor width should reach 45 feet, but that requires some distance to accomplish from a station location. In Houston, north of I-45, its station on the first block, station on the second block, median on the third and fourth blocks, station on the fifth and sixth blocks, etc. That's an equivalent station spacing every four blocks. That doesn't leave much room to narrowing down to 45 feet then expand out to 55 feet again. That's why they don't do it in Houston. It's probably doable in Austin if station spacing was increased. But will 45 feet be enough considering Main Street in downtown Houston is perfectly straight, Guadalupe and Lamar Streets in Austin aren't. It'll require more clearance between the tracks because of the curves, although I don't know how much.
Looking at Guadalupe Street between MLK and 30th Street in Austin, all the lanes are generally 10 feet wide, including turn lanes and parking lanes. Where the street is 4 lanes wide, 40 feet. Where the street is 5 lanes wide, 50 feet (4 traffic lanes plus 1 turning lane or parking lane). Where the street is 6 lanes wide (4 traffic lanes, 1 turning lane, and 1 parking lane), 60 feet. Most of the route will support 2 traffic lanes and 2 tracks in the center,
not all. None of the route supports any standard platforms within the street between the curves with tracks running down the center of the street. The train platforms will have to be outside the existing curbs, meaning placing the tracks in the outside (right) lanes. Exactly what bus, rapid bus plans and streetcar lanes/tracks will be located.
There aren't many places along the route where the street can be widen, although I believe the 40 feet sections can be widen to achieve a width of 45 feet for center running. Moving the tracks from the center to the outside lanes of the street only saves around 5 feet, the space needed for clearance between the trains and catenary poles.
South of MLK in downtown Austin there's plenty of room for dedicate track lanes, north of 29th Street there is to, with 60 feet to work with. But even at 60 feet street width, you wont't be seeing 4 traffic lanes and 2 tracks with stations in the center of the street until you reach Airport Blvd. North of Airport Blvd, Lamar's width increases to 80 feet. So, between MLK and Airport, were discussing 2 center running tracks and 2 traffic lanes. Or 2 tracks in the outside lanes with 4 traffic lanes in the center, with station platform placed outside the curbs on the existing sidewalk locations. Only north of Airport can you keep 4 traffic lanes and 2 center running tracks with station platforms between the existing curbs. There's 1 mile between MLK and 29th Street., where there are significant challenges providing dedicated center lanes for tracks.
Considering light rail will probably run in an outside "right" lane on the one-way street couplet in downtown Austin for 1.25 miles, in the outside "right" lane an additional mile in "The Drag", and an additional 3.25 miles in the outside "right" lane up to Airport, is light rail the best solution for Austin? It's 5 miles running in the right outside lane to maximize 4 traffic lanes as much as possible, still with just 2 traffic lanes in "The Drag" in the center of the route. I'm thinking that it might be best to share the right lane with traffic over just this one mile, and minimize the stations here as well to keep traffic moving as much as possible, even if that means the trains are delayed some. Otherwise, if you need dedicated lanes here as well for the trains, it's time to think going up and over the traffic for this mile.
Looking in the southern direction, South Congres minimum width is 70 feet wide. There are locations where it's 80 and 90 feet wide when including parking spaces. Tracks in the center with station platforms and maintaining 4 traffic lanes will be far easier to implement.
Of course, there's always the option of widening the streets by spending lots of money to buy the expanded right of way. An additional 20 feet would be nice in "The Drag" one mile section so 4 lanes can still be used by traffic while dedicated lanes for 2 tracks.
Another out of the box solution to consider is to single track that one mile of "The Drag". Not sure it will help much with traffic, but leaves options to have 3 lanes for traffic, 2 through lanes and 1 turning lane. Even averaging a slow 10 mph for the one mile, it'll only take 6 minutes for a train to transverse that mile, that would support up to 10 trains with 12 minute headways in both directions. And I'm sure a train could average faster, like 30 mph, in a dedicated lane. That would lower the elapse time over that mile to 2 minutes, allowing up to 30 trains with 4 minute headways. This would make this one mile the choke point of the rail system, but it is going to be anyways.
By the way, that drawing of yours is just a line on a street map. A preliminary engineering drawing looks like what in this link for the Oak Cliff streetcar project changes.
http://www.dart.org/ShareRoot/about/...lAppendixF.pdf