The building of the railways created jobs.
It helped the agricultural and industrial expansion.
It helped transport of raw materials.
(As they have said.)
It also helped them win the WWII (by moving troops from Vladivostok to the Front), it helped them deport millions of people, so jobs could be created for other people while these people were at the Gulag, etc.
I know this is morally wrong, but it still helped them someway.