Question

These days
all trains are crowded.(nagative)​

Answer 1

THESE DAYS ALL TRAINS ARE NOT VACANT.

HERE, NOT VACANT IS USED AS WHILE CONVERTING FROM AFFIRMATIVE TO NEGATIVE ANTONYMS SHOULD BE USED.

VACANT IS ONE OF THE ANTONYMS OF CROWD.

HERE , THIS SENTENCE MEANS THAT - IN PRESENT TIMES THE TRAINS ARE FILLED WITH PEOPLE . SO , THE BEST OPINION IS TO USE VACANT.

HOPE IT HELPS YOU....