The SMS is to be sent to the number 139, according to the official Twitter account of Ministry of Railways~CHECK~ @RailMinIndia.
Here are the various types of SMSes that need to be sent for specific inquiries:
If you want to enquire about the status of your ticket, you need to type PNR <10-digit PNR number>.
For knowing about train arrival/departure, you can type AD .
For enquiring about the current train location, type SPOT .
To check the seat availability in general quota, type SEAT
To check the availability of seats under Tatkal scheme, type TSEAT .
To check the name of your train or number, you must say: TN or TN .
To know the time table, type TIME .
To know the next train, type NEXT .
To enquire about the train route, say: ROUTE .
In order to enquire about fare details, you can say: FARE .
Class field is specified as follows:
1A - First AC
2A - Second AC
CC-AC Chair Car
2S - Second Sitting