Speed

Los Angeles SWAT officers Jack Traven and Harry Temple thwart an attempt by a bomber to hold an elevator full of people for a $3 million ransom. They corner the bomber, but as he grabs Harry, Jack shoots Harry in the leg, forcing the bomber to release him. He turns and runs around a corner, apparently dying in an explosion. Jack and Harry are praised by their superior, Lieutenant "Mac" McMahon.
The next day, Jack sees a city bus explode. The bomber then contacts him, explaining that a similar bomb is rigged on another bus, and will get activated if it goes over 50 miles per hour (80Â km/h) and blow up when the speed drops below 50 mph. The bomber demands a larger ransom of $3.7 million and threatens to detonate the bus if passengers are offloaded. Jack races through traffic and manages to board the bus, but it has already gone over 50 mph. Jack explains the situation to the driver Sam, but a panicked small-time criminal on the bus fires his gun, wounding Sam. Another passenger Annie Porter takes Sam's place behind the wheel, while Jack explains the situation to everyone aboard. Jack examines the bomb under the bus and phones Harry, who uses clues to identify the bomber as a former police officer.
The police clear a route for the bus to the unopened 105 Freeway. Mac insists they offload the passengers onto a pacing flatbed truck, but Jack warns about the bomber's demands. The bomber allows the officers to offload the injured Sam, but detonates an explosive under the bus's front stairs when another passenger attempts to escape. Jack learns that part of the freeway ahead is incomplete, but forces Annie to accelerate the bus and jump the gap. Jack directs Annie to nearby Los Angeles International Airport to drive on the runway. Meanwhile, Harry has identified the bomber as Howard Payne, a retired Atlanta bomb squad officer with a local address, and takes a SWAT team there. However, the house is rigged with explosives and explodes, killing Harry and his team.
Jack attempts to defuse the bomb while riding on a towed sledge under the bus, but it hits debris on the runway and he can only hold on by puncturing the fuel tank. After being pulled back aboard by the passengers, Jack learns that Harry has been killed and that Howard has been watching the bus the entire time with a hidden camera. Mac gets a local news crew to record a transmission, then rebroadcast it in a continuous loop to fool Howard while the passengers are unloaded onto an airport bus before all the fuel leaks out. The bus suffers a flat tire, allowing Jack and Annie to escape via a floor access panel. The empty bus then drops below 50 mph, causing it to explode as it reaches an empty 707 cargo plane.
Jack and Mac continue the ruse and head to Pershing Square to drop the ransom. Realizing he has been fooled, Howard poses as a police officer and seizes Annie. When the drop is made into a waste can, Jack discovers a hole under the can leading into the Metro Red Line subway. Jack finds Howard and Annie, but she is wearing a vest covered with explosives and rigged to a pressure-release detonator. Howard hijacks a subway train, handcuffs Annie to a pole, and sets the train in motion while Jack pursues them. After killing the train driver, Howard attempts a bribe with the ransom money, but is enraged when a paint bomb in the money bag goes off. He and Jack engage in a fight on the roof of the train until Howard is decapitated by an overhead signal. Jack removes the vest from Annie, but she is still handcuffed. Since they cannot stop the train, Jack instead sets it for full speed, causing it to derail through a construction site and onto Hollywood Boulevard before coming to a stop. Jack and Annie both survive and kiss passionately while onlookers take pictures.

Question: What is Jack riding on as he tries to defuse the bomb?
Answer: a towed sledge under the bus