I wonder how many trains per day use this section of track.
There are 18 trains on weekdays through Fallowfield. Seven Toronto-Ottawa round trips (several are to/from Montreal) and two trains that originate/terminate at Fallowfield.
The warrants for a grade-separation crossing must depend on the traffic volumes and on the train volumes. I have heard "hindsight is 20-20" people talking about the City planning a grade-separation but the cost was excessive ($80M). I would guess that grade-separation makes more sense were trains frequencies are higher than a few Via trains per day. Along commuter rail lines, it makes more sense since there are now probably close to 50 trains per day on the Lakeshore line. If you think of a train the same as an intersection with cross traffic - even 50 is not that large of a number. That is why if any rail line gets down to 5 or 10 minutes frequency, grade-separation is required.
There are 13 rail crossings that I can think of where TTC buses cross at grade in Toronto, not counting minor industrial spurs such as those on Belfield Road or those in York Region (Markham Road, McCowan Avenue, or those on Route 107).
By far the busiest rail crossing that the TTC operates across is CN/Metrolinx Kingston Subdivision at Morningside Avenue. There are 90 scheduled GO Trains most weekdays, plus 24-26 VIA trains weekdays (not counting two eastbound joined trains) and the occasional freight - about 114-120 trains each weekday total. About 400 buses on route 116 cross here on weekdays.
The busiest in terms of buses crossing a railway is the newest grade crossing, the CN/Metrolinx Newmarket Subdivision at the York University busway. 453 TTC buses on Route 196 cross it one-way during school year weekdays, plus 38 Viva Orange buses. (So just under 1000 daily bus crossings). However, there are only 10 daily GO trains, plus the occasional freight and the outbound Canadian, 2-3 times a week.
Steeles Avenue and Finch Avenue grade crossings with the CN/Metrolinx Uxbridge Sub are also very busy bus crossings (routes 43/53/353 and 39/199/308) - there are 14 GO train movements at these crossings.
The others are at Cummer Avenue (route 42), Huntingwood Drive (169) Danforth Road (route 16/302), Strachan Avenue (route 63/316), Church Street (route 59), Oak Street (route 59), Carl Hall Road (route 101), Wallace Avenue (community route 402) and Steeles near Staines Road (53A). Strachan and Church will soon be grade separated.