Some thoughts
Ulaan Ovoo is about 260 miles from Ulaanbaartar and a rail line runs north from Ulaanbaatar into Russia. I recall reading that Ulaan Ovoo is was about ~120 km from rail and believe that it must be the same rail line.
Is it possible that Mongolia has already decided that Ulaan Ovoo's coal will be initially used to Power Plants serving Ulaanbaatar and it's vicinity and for Russian Power Plants in close proximity to the Mongolia's border..