http://www.ssa.gov/pubs/EN-05-10045.pdf I had to dig to find the specific publication. I am perusing it now myself. After reviewing the publication, it does not appear to clarify the determination for OP's circumstances either way.
I am familiar with that publication and it has absolutely nothing to do with OP's situation. There are organizations, some police departments, some firefighters, career military and other professions where the employees do not pay into social security but rather their pension replaces social security to one extent or another. In those instances they do not earn separate social security credits on their wages, and social security is not withheld from their wages. Therefore people in those professions who have not earned social security credits from other employment, either before or after those careers, would not be eligible for SSDI or regular SS benefits.
In other words, existing SSDI benefits would NOT reduce because they began to collect those pensions. Any SSDI benefits would be based solely on their earnings credits from before or after the other career.
Railroad employees have a mixture of Social Security and pension.
Someone receiving a pension from a wholly private company, like Hostess, would not fall under that publication. Since SSDI is not means tested, another pension would not reduce SSDI benefits...although its likely that someone receiving a private company pension would have already been switched from SSDI to regular SS benefits anyway.