Inivitation Letter ( Latest One Provided by company Stating Purpose and Duration of Visit)
Training Schedule (Training details for all Duration of Stay)
Business Card ( not Mandatory but it can be asked, It was asked for me twice)
4.Company letter stating that You are Going for US visit on Business Purpose and descrition of Project for which u wil be trained) ( Invitation letter and this both are different)
Also, carry the print out of the return ticket. This may not be required, but good to carry the appointment letter and some pay stubs. They would NOT ask for these, but just some additional evidence to carry.