In particularly demanding situations like High Speed Rail, or busy and curvy freight routes with heavy lateral forces on the rails, or subways where the ties pretty much need to be replaced all at once, or in slab track, then concrete is clearly the way to go.
On a straight route that will see only relatively lightweight EMUs at moderate speeds, where the tracks are in ballast and neither tunneled nor elevated, periodic inspection and replacement of deteriorating ties as needed is not that difficult. This means the lifecycle costs of wood will be lower.
It really perplexes me why concrete ties are used in some situations where there's really no point... but as long as the feds are paying, why not, right?!
(image credit:
lightrailnow.org)