To wander in search of work. Hence tramping, on tramp
"Tramping" was well established and regulated among artisans of all sorts including stonemasons, carpenters and bricklayers who made up substantial sections of the workforce constructing railways [i]. The navvies who moved the rocks and soil were called Excavators. They seem to have developed an informal practice of funding one of their number (giving him a "tramping bob") to go in search of work when a job was coming to an end or if they were unhappy with their current employer. [ii].

References:

[i] Hobsbawn, Eric. Labouring Men: Studies in the history of labour (Wiedenfeld & Nicholson, London, 1964) pp 34-63
[ii] Brook, David. The Railway Navvy 'That Despicable Race of Men' (David & Charles, Newton Abbot, 1963) p

039, 306, 263, 335