Why would a girder bridge structure for a rail line have only one section with through girders?

A rather high rail line trestle over US75 in Plano Texas has a whole series of girder beam sections where the girders are under the rails then one section where the girders are beside the rails. The obvious reason in looking is that the clearance under the section is much higher and is it placed over the DART Rail beside the highway. But is it that much cheaper and easier to put the beams under that they couldn't do the through beam all the way? The example is here 32掳58'0.51"N 96掳43'15.71"W and on Google maps or Earth you can barely make out the shape of the shadow. And Wow - here it is on Google Street View via Tiny URL http://tinyurl.com/cacgedWhy would a girder bridge structure for a rail line have only one section with through girders?
Because the engineers in their infinite wisdom decided this is safe and it meets the budget for this project.