The CDL Doubles and Triples endorsement (T endorsement) is required to operate commercial vehicles towing two or three trailers simultaneously. It's one of the more specialized CDL endorsements — and can open doors to higher-paying freight positions.
When Is the T Endorsement Required?
You need the T endorsement when:
- Pulling a set of doubles (two 28-foot trailers) or triples (three 28-foot trailers)
- Operating any combination vehicle with more than one trailer
Doubles/triples are most common in LTL (less-than-truckload) freight, package delivery, and some regional trucking operations.
Test Format
- Questions: ~20 questions (varies by state)
- Passing score: 80%
- No separate skills test required — the T endorsement is knowledge-only
Key Test Topics
Coupling Doubles and Triples
Coupling multiple trailers requires connecting a converter dolly between trailers. Know the proper sequence: couple the second trailer to the dolly, then couple the whole unit to the first trailer. Always inspect all connections before moving.
Converter Dolly
A converter dolly is a small set of axles with a fifth wheel used to connect additional trailers in a double or triple combination. It must be inspected like any other vehicle component — including lights, tires, and coupling hardware.
Rearward Amplification
Rearward amplification ("crack the whip") is more severe with doubles and especially triples. The last trailer swings much wider than the tractor when you change lanes or swerve. Drive smoothly and avoid sudden steering inputs.
Off-Tracking
Off-tracking is greater with longer combinations. Wide turns are necessary — but make sure you don't drift into other lanes.
Brake Timing
With multiple trailers, braking forces are unequal. The rear trailer receives braking signals slightly later than the front, which can cause the combination to compress and become unstable. Apply brakes gradually and well in advance.
Who Hires Drivers with a T Endorsement?
- FedEx Freight — extensively uses doubles combinations
- UPS Freight
- ABF Freight
- Most LTL carriers that operate on the Western US double/triple legal states
Note: Doubles and triples are not legal in all states. The Western US (especially the "long combination vehicle" states) are the main markets.