The Tinsukia Mail history you posted is not correct. Tinsukia Mail was introduced after Farakka Barrage was opened. The only Express train that took the railway steamer from Farakka to Khejuriaghat across the river was Darjeeling Mail.Khejuriaghat and Malda Town stations were on North East Frontier Railway. Malda Town was a small station on this route. See this time table from 1966. There is no Tinsukia Mail. I also checked the time table till 1971 and no Tinsukia Mail.

See this prior to 1971 arrangements

See this prior to 1971 arrangements

Magadh Express was introduced in 1984. Before that, there was 191/192 Sonbhadra Express (2 days from Patna, 1 day from Bhagalpur), and 167/168 Vikramshila Express (2 days from Bhagalpur).In 1984, these 2 trains were merged to give a daily train from Bihar to Delhi. See the timetable from 1983.