My limited understanding of snakes is from a murderer's perspective - I have killed many during my childhood days. I have seen that snakes like to bask in the sunshine. During my childhood days while returning home in the night, with non-existent streetlights, we used to literrally stumble upon snakes on the road who lie there to get the warmth of the road.

I have seen them after rains, and also in winter when they prefer to lie on the puckka roads. I have seen that in the jungle roads as well. Especially in BR Hills, in one trip we found many on the road and the jungle lodges vehicle even ran over one cobra.

Snakes being poikilothermal (cold blooded) need to warm themselves.

As far as Shaktipada dada's experience of snakes lying in railway lines, it can also be that the metal track heats up and gives them the warmth.