Many websites and history books report that (example ) but I couldn't find any documentation of the time (photos, newspaper articles, etc...)
According to the World Diary 1929-1934 at page 35:
On April 5  Gandhi carried out his threat... Within a week British soldiers throughout India were using their swagger sticks to flick the white cotton caps off the heads of Gandhi's supporters, who were obstructing traffic by lying down on railway tracks and public thoroughfares...
Similarly, according to the New International Yearbook at page 365:
His act of open defiance of the Government took place on April 6...railway employees quit their work and in some cases lay across the tracks to obstruct traffic
See also Demand for Special Train: Nationalists Lie down on Line The West Australian 19 March 1940 page 8
200 Nationalists lay down on the railway track in front of a train near Calcutta