The correct English translation of “যাত্রীরা যখন ট্রেনে। উঠা-নামা করছিল, তখন আমি ব্যাগটা হারিয়েছিলাম’ is:

While the passengers were getting and getting up the train. I lost my bag

While the passengers were getting down and getting in the train, I lost my bag

As the passengers climbed up and down the train, I had lost my bag

While the passengers were getting down and getting in I was losing my bag

The passengers got up and down the train and I lost my bag'