Chalk Marks Question

Dean ONeill

I have a question(s) on chalk marks. As I look at old photos, it appears there are two "thickness" of chalk marks. 
Chalk marks made with a THIN piece of chalk, much like a school room piece of chalk. Roughly .25 inches thick or less. 
Chalk marks made with a THICK piece of chalk, much like kiddie sidewalk chalk of today. Roughly .5 inches thick. 

Any speculation on which was used under certain circumstances, or was it completely random?
Did certain RRs have a preference?
I seem to also remember from decades back a kind of non-chalk writing stick, more oily, that was more durable. Were those also used?

Dean ONeill
Redmond WA

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