Yes, STRAIN is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, R 1, A 1, I 1, N 1.

Is it a valid word?

STRAIN is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

(physics) deformation of a physical body under the action of applied forces
difficulty that causes worry or emotional tension
a succession of notes forming a distinctive sequence
(psychology) nervousness resulting from mental stress
a special variety of domesticated animals within a species
(biology) a group of organisms within a species that differ in trivial ways from similar groups
injury to a muscle (often caused by overuse); results in swelling and pain
the general meaning or substance of an utterance
an effortful attempt to attain a goal
an intense or violent exertion
the act of singing

to exert much effort or energy
test the limits of
use to the utmost; exert vigorously or to full capacity
separate by passing through a sieve or other straining device to separate out coarser elements
cause to be tense and uneasy or nervous or anxious
become stretched or tense or taut
remove by passing through a filter
rub through a strainer or process in an electric blender
alter the shape of (something) by stress

Also: melodic line, melodic phrase, mental strain, nervous strain, var., tense up, separate out, filter out.