I searched "pickaxe rail mine" and was unable to find this bug reported already. Any well made report documenting this should've come up with this search.
Every type of rail drops if mined by hand, though this is inconsistent being that they are all predominantly made of metal.
Rails, activator rails, and detector rails should require a stone pickaxe to mine as they are made mostly of iron. Powered rails should require an iron pickaxe to mine as they are made mostly of gold which requires an iron pickaxe to mine.
This may also extend to other metal-related blocks, but these are the only examples I have found so far.
This may be intended.