If you're not going to print this very large, you could use a wide angle 14mm / 16-35, etc. and put them in the middle of the frame (to avoid distortion), then crop out the bottom 1/3 to 1/2 of the photo, but given the type of photo (dramatic shot of a special memory), it needs to be high-res for a big print. If distortion is not as much of an issue, you can get close, tilt up and have the arch & railroad converge. Google ultra wide angle wedding photos or something like that for some examples (like this - scroll down to the three 16-35 examples). That way you still get the full frame.
Another option is to use the TS-E 24 and the shift function to shift, rotate, and shift again for a full panoramic effect. Or simply use shift to correct the verticals and you still have a great full frame shot.
A third option is to stitch, like you say, possibly using the Brenizer effect with a longer lens. A lot depends on how far you can get from the arch - and if that's not the limit, communication with the couple will be an issue.
Just my 2c - there are plenty of ways to shoot this - I'd recommend trying at least 2 to 3 ways and letting the couple pick out their favorite.
Those 16-35 shots are perfect. It was good enough to get the height of the object into the scene, the only draw back is that he had to be very far away.
My goal is to get the full height and most of the width of the train tracks, but also close enough to get good detail on the couples face.