adma
Superstar
Some of the bridges we had at the fringes of the city were Bailey bridges, and I gather they were usually meant to be temporary. There's another one on the abandoned track of Don Mills Road as it descends into the valley; it used to carry traffic one lane at a time over the CN line. It probably replaced a wooden structure; my guess is it was put at in an era when there was already planning in the works for something more permanent.
I'm not sure whether that particular Bailey actually post-dates the Don Mills diversion, i.e. built strictly as a bikeway/pathway affair (though yes, it does follow the old Don Mills path). And even if it (or a presumably wooden predecessor) did serve Don Mills, remember that we're dealing with a road/rail grade separation, as opposed to a river crossing. As far as I can tell, there was greater leeway for "impermanent" construction with road crossing rail, esp. in rural areas--and you can still find a lot of wooden road-over-rail bridges in rural Ontario (eg. the old Danforth path btw/Newcastle and Port Hope).
Remember, too, that Bailey bridges were a WWII invention, and they have nothing to do with something like, say, Flindon. (Another Toronto Bailey, interestingly enough given another current thread, was that which originally carried Indian Line across the Humber below Claireville Dam.)
Much more typical of river crossings between the wars is just to the south, the concrete arch that originally carried Don Mills over the Don. And take note: Don Mills was just as "minor" as Albion/Flindon, if not more so...