We could just see the train approaching in the distance.
This sentence has been taken from Cambridge Dictionary. To know the meaning of "approach" I searched in this dictionary and I found this sentence.
If this sentence would be written in the following way, it would seem natural to me.
We could just see (that) the train was approaching in the distance.
I cannot understand why "was" has been omitted. In which cases this sort of omossion is allowed?