Sounds like an excellent route for light rail, or regional rail to me.
Regional rail would be far less expensive than light rail to build and operate, mainly because the service frequency between headways is 20 to 30 minutes, vs every 5 to 10 minutes.
Dart builds most of its light rail lines over abandoned freight tracks. Dart's costs are around $50 million per mile, including the purchase of the right-of-way. For significantly less, around $5 (CapMetro) to $15 (DCTA) million per mile, regional rail can be built. The difference between CapMetro's and DCTA costs can be attributed to DCTA having to replace all the tracks, while CapMetro is continuing to use most of the existing tracks.
A significant cost for light rail is running electric lines along the tracks. NCTCOG estimate $1.6 million per mile per track. A double track light rail line cost $3.2 million per mile just to electrify.
New single track, all you'll need for regional rail, costs around $1 million per mile. You'll have to pay $2 million per mile for double tracks, whether for light rail or regional rail.
Then there's new "Quiet Zone" signal installations all along the route, costing up to $.5 million at each intersection. And that doesn't include the costs for the trains and stations.
Downtown San Antonio to Loop 1604 is approximately 13 miles. At $50 million per mile for light rail, total construction costs should be near $800 million. At $15 million per mile for regional rail with all new tracks, total construction costs should be near $195 million. At $5 million per mile without new tracks, total construction costs should be near $80 million. Of course, whether San Antonio needs to lay new tracks depends upon the condition of the old tracks, but I would plan on doing so before starting service to provide a great first impression and the smoothest ride.
Without any freight, or very low number of freight trains a day, the option for using cheaper FRA non-compliant DMU trains is a good possibly.
Like CapMetro's Stadler GTWs, or NJT River Line Stadler GTWs trains.
CapMetro Rail
NJT River Line
Note: The River Line GTW train is smaller than CapMetro's, and can turn tighter curves with a minimum turning radius of 130 feet, vs CapMetro's 300 feet. Most real light rail trains minimum turning radius is 82 feet, and most streetcars are 50 feet. How tight your train must turn depends upon whether they plan to run the trains down city streets downtown around corners.
Other DMU's used in North America:
Siemens Desiro by San Diego Sprinter
Bombardier Talent by Ottawa's O Train
Colorado Railcar DMU by Florida's TriMet
There's plenty of light rail vendors supplying light rail cars in North America. But I suggest San Antonio buy DMUs, and save significant funds constructing electric lines to power the trains. Dart and Metro require a full cent sales tax to support running light rail trains. DMUs can be supported by a half cent sales tax. How much funding San Antonio is willing to tax itself will determine what San Antonio will build.