See the 2009-current experiment known as
TriMet's Westside Express Service, or WES.
Step 1: Someone shows off a fancy railcar and says how great it is. (oooh, look, a British DMU, we have to buy this European design!)
Step 2: Local governments/transit agencies search in vain for some rail line to run the car on, with zero regard for true transit planning. (I think there's a rail line here that isn't used much, maybe we can run trains here again?)
Step 3: After blowing the original budget by 200%, the line opens to mass media hype and fanfare, which quickly fizzles out when the public realizes the service is hardly feasible. (Yay, publicly funded party with all the elected officials that drove up - and home - in their SUVs!)
Step 4: We wonder why transit ridership is in the gutter when we've spent $165 million to build the system, another $30-40 million in capital expenses afterwards, $16 million a year (times 12 years) to actually pay to run the system...but because we blew all this money we had to cut the transit service people actually did use, resulting in people being forced to drive. Because buses suck and trains are the best thing since sliced bread, even if those trains don't go anywhere people want or need to go, whereas the buses did.