Doepack has a good insight. I've been to Houston a lot and rode the light rail. It's almost as big as Chicago, but very spread out. There are only 3-4 rail lines and they don't go a lot of places - no airports especially. As you can guess, ridership is pretty light. It's a combination of not having a comprehensive network and being in a spread-out city. There are effectively 4+ downtowns.
GIven the above, I assume the free rides are a way to enlighten drives to the benefits of light rail. Unfortunately, I don't have high hopes because the free ride doesn't change the abbreviated nature of the network. If one gets a free ride, but has to drive fifteen minutes to the station and circle the parking lot for ten minutes to find a spot, to what benefit? For me, urban transit railroads are useful when I can walk right into a station (IE 5-10 blocks), board a train in 5-10 minutes, and either arrive at my destination in roughly the same time as traffic OR realize a savings in parking, gas, costs, etc that makes it worth the extra time. Few people, even sometimes railfans, want to take a train for the sake of taking a train.
Dig the new rr.net Instagram account: @railroad_dot_net