Step 1: Picture what undulating actually demands from a haulage system.
An undulating roadway rises and falls repeatedly along its length, unlike a roadway with one continuous gradient. Any haulage system used here needs positive control of the tub train in both directions, pulling it up the rises and holding it back (braking) on the dips, since gravity alone cannot be relied upon consistently.
Step 2: See why the simpler systems fall short.
Direct rope haulage only pulls the train one way, uphill, and lets gravity bring the empty tubs back, which only works on a single, continuous gradient. Gravity rope haulage depends entirely on a specific downhill profile to work at all. Endless rope haulage is best suited to fairly uniform, gentle gradients and does not offer strong, independent control from both ends of the train.
Step 3: Explain why two ropes solve the undulation problem.
Main and tail rope haulage uses one rope attached to the front of the train, the main rope, to pull it inbye, and a second rope attached to the back, the tail rope, run around a pulley to pull it outbye or hold it back. Because the train is always attached to a powered rope at both ends, the hauler can positively pull it up any rise and control its descent on any dip, whatever combination of ups and downs the roadway contains, which is exactly the flexibility an undulating roadway requires.
\[ \boxed{\text{Main and Tail rope haulage}} \]