Those who worry that the plan to build a high-speed rail line between Houston and Dallas by 2021 is overly ambitious were probably having heart palpatations after the seven-member Commission for High-Speed Rail in Dallas/Fort Worth expanded its vision for bullet trains in Texas during a Wednesday meeting in Dallas.“The Houston-to-Dallas connection is going to happen,” Ted Houghton, chairman of the Texas Transportation Commission, confidently told Star-Telegram reporter Gordon Dickson.But even this initial line — which will now include a stop in Bryan-College Station — is just the tip of the iceberg.The commission unanimously agreed Wednesday to pursue federal funding to study the possible construction of a Fort Worth-Austin connection.Of course, a Fort Worth-Austin connection is only possible after officials figure out how to finance Dallas-Arlington-Fort Worth link — a top priority for the residents of Tarrant County. Unlike the Houston-Dallas route, which will cost about $10 billion and be privately financed by Texas Central Railway, the Metroplex connection will probably rely on public funds. Exactly how the state will come up with the estimated $2.5 billion to $4 billion needed remains to be determined. Former Fort Worth city councilman and high-speed rail commission member Bill Meadows told the Star-Telegram editorial board in June that a $15 million federal grant to study high-speed rail will be devoted entirely to the Metroplex link. But coming up with a plan to fund the line to Arlington and Fort Worth wasn’t the commission’s focus this week. Instead, its members explored the possibility of studying high-speed-rail lines connecting Oklahoma City to Austin, San Antonio and even Monterrey, Mexico. Only a few years ago, the idea of a bullet train zipping between Dallas and Houston at 220 mph was little more than a pipe dream. That the high-speed rail commission is now considering expanding its lines beyond the border is a sign of the progress made. Still, for Tarrant County, ensuring that the bullet train serves the entire Metroplex is the first order of business.