All Trailers in this series come with UFP® Torsion Axles with their Vault Lubrication System.  All trailers in this series come with Disc surge brakes on two axles except  21K  comes with surge disc brakes on (3) axels

ATT- TRI 3234 - 3638 - 3640
ATTRI 3224 GROSS WEIGHT 12,000 I-Beam 6 1/2"  Carrying Capacity 10.3K
ATTRI 3638 GROSS WEIGHT 15,000 I-Beam 7 1/2" Carrying Capacity 12.7K
ATTRI 3640 GROSS WEIGHT 18,000 I-Beam 8" Carrying Capacity 15.02K
ATTRI 3621 GROSS WEIGHT 21,000 I-Beam 8" Carrying Capacity 18K

Electric over Hydraulic is a good option
Standard on 21K Trailers.