The first thing you should know in this case are the following conversions:
1T = 2204.62 lb
1mi = 5280 feet
Applying the conversions we have:
6 T / mi * (1/5280) (mi / feet) * (2204.62 / 1) (lb / T) = 2.50525 lb / ft
answer
6 T / mi = 2.50525 lb / ft