Also, didn't the inevitable small lineside fires that still occurred create 'fire breaks' and kept vegitation down?
I was on the North Norfolk Railway in August 1977 on the footplate and even with a spark arrestor, there were still regular fires of a particular farmer's field. One time we stopped to try and put the fire out pending the fire service turning-up. The farmer seemed to have a 'sixth sense' about his fields as he seemed to produce himself at Sheringham to demand his compensation before the trains had arrived back itself! I was told at the time that in steam days it was a fact-of-life that lineside fires occurred all the time. I also read in a railway mag a few years after steam was removed from the main line that a pair of hard-working diesels had started a lineside fire on a gradient, presumably from sparks from the traction motors.
