No meu "standing" entender significa "parado", sem movimento.
One of the senses the Thesaurus provides is that of "motionless/not moving/at a standstill/immobile/unmoving/stationary, etc...
So, it makes sense that the train is not moving yet, it is waiting for passengers to get on.
By the way, sentences with the words "train" "standing" "on platform" are rather common in English, at least the British variety.
In another case, in physics, standing can be understood as "placed" (situado, posicionado, etc ) as we can see from an example taken from the following exercise statement:
A train standing at the outer signal of a railway station is blowing a whistle of frequency 500 Hz, the speed of the sound is (340 ms)^ -1.
(1) find the frequency of the sound of the whistle heard by a man standing on the platform when the train (a) approaches the platform with a speed of (20 ms)^ -1 and (b) recedes from the platform with a speed of (20 ms)^ -1.
But then, this second case is from another context there.
Normally to talk about trains sitting on a plataform would mean serious problem, impatience or that it being out of order.
Anyway, as English is surprisingly in many ways, I will leave more advanced experts chime in and enlighten us further on the topic.
A few feet away from her on the platform, 24-year-old Jin Rui leaned against a railing, headphones jammed in her ears. She had been waiting on the platform for 20 minutes, and a train had been sitting on the platform the entire time — but it was so crowded that there was no room for her to board.