Just to add to @MadeleineAllbright 's excellent summary.
When India was partitioned Pakistan was two areas known as East and West Pakistan with 1000 miles of India in between.
East Pakistan was Bengali speaking but Pakistan imposed Urdu on them, this is one of many things that made East Pakistan declare itself an independent Bangladesh.
Also the violence was on a scale I doubt has been seen before. 10 million refugees were travelling from India to Pakistan and visa versa. Most people travelled by train. Many trains were stopped, the drivers held at gunpoint.
The trains were the type with carriages you accessed from the platform so there was no where to go. People were massacred, they were refugees, many women and children crowded in to the carriages with no escape.
Once the attackers had finished the train was allowed to continue on its journey, they became known as 'blood trains'.
I don't think it is helpful to say which train was the first because it happened across the borders in both ways.
It is estimated over a million people were killed in this way, or in riots or other violence. Thousands of women / girls survived the massacres because they were abducted. After partition both India and Pakistan found abducted women but most refused to return believing their families would no longer accept them.
Meanwhile the British government pretended it was a great time of independence and hand over.
The final irony is that many of those who survived emigrated to the UK. Some of the stories are here https://www.bbc.co.uk/programmes/b090rrl0