I'm saying that the actual Houston/Galveston traffic between the two isn't at all worth investing in rail. It's not like The Woodlands, where there's solid traffic on I-45 AND an auxiliary toll road out to the area. In all my trips to Galveston or parts nearby, I-45 is bad (which may or may not relate to construction) but by Texas City it's not bad at all. If road traffic was an issue then when they rebuilt the two Galveston I-45 spans (one of which partially dated back to the 1930s) they would've been much wider. The spans remained at three lanes in each direction with a very long merging lane from the entrance of Harborside Drive (TX 275)/Teichman Road and Tiki Island (and vice versa), presumably with the intent to add a fourth lane if necessary. The other reason why the bridge was rebuilt was in favor of ships. The old bridge and railroad bridge were declared a "hazard" as of 2001. The old bridge had a width of 125 feet and the rail 120 feet. The rebuilt bridge had a clearance of 300 feet, with the rebuilt rail bridge also having 300 feet.
From the fact that the default bridge position is "raised", there's a clear ship bias in Galveston. The railroad in Galveston basically functions as a long rail spur, with storage, a few ports, and a few minor others (like the railroad museum). The rails east of 28th Street are also hardly used (if ever), especially since it goes through a highly tourist area. The only reason people still talk about rail to Galveston is that the infrastructure exists, but when inconvenient facts start revealing themselves (unless an entirely new bridge was built, the commuter rail will have to stop at the Causeway, which is probably why the 35 mph cap exists). And when you're talking about a new bridge, any cost-savings on the existing infrastructure vanish (even if UP was totally cool with the passenger trains being on it, which is doubtful). Since I-45 traffic on the bridge is already comparatively light, the question of how much of that traffic is originating from anywhere close to downtown Houston and not the entire region (including as far as Dallas and Louisiana).