As with all old passenger engines, it is not hard to find accounts of old heads swearing that T-12's could go 60 moph (a mile per minute.) I have my doubts.
OTOH, since they held down schedules that averaged a bit over 20 mph (trains 317/318, Salida-Alamosa, 317 averaged 24 mph Villa Grove-Alamosa) and were carded for around 27 mph between several station pairs, I would make the guess that 40 mph or bit more on a reliable basis. They'd have to have been capable of that to hold the schedule. Maybe even as much as 50 mph, depends on if you prefer 2/3 or 1/2 as the fraction of max speed a passenger train can be expected to average. Schedule had flag stops every 5 or 6 miiles and regular stops at Hooper, Moffat & Villa Grove (every 18-20 miles.)
hank