http://www.eeoc.gov/foia/letters/2004/ada_inquiries_examinations_2.html
this speaks to what you are asking in a convulated lawyer way.
Basically, they can ask you for documentation sufficient to substantiate a sick call. I am very surprised that a union would allow this to be placed in there contract...however, whatever rights you have (federal law is vague, however your states laws is probably tougher.)
Additionally, I would talk to a manager in HR as the enforcement of this could expose the company to some liability under HIPAA/ADA etc.
Making you get a doctors note could be interpreted as requiring a medical examination that ADA would prohibit unless some circumstances exhist to give the employer reasonable cause...yadayadayada
Another snag could be HIPAA (medical records privacy)For example, if the employer has medical records about you they are required to keep these in a seperate file with limited access seperate from your personelle/training records. In other words it can't be the same file that the training department is going to look at...or the payroll clerk...etc.
This kind of stuff is usually complicated/convoluted enough to make most HR types settle for a doctors note saying you were sick and be done with it.
You are correct that whatever rights you are given under laws cannot be taken away with a contract. It's the hirearchy of laws...constitution, statues, ..., ..., then somewhere at the bottom is contracts. A lower form of law (a contract is one) cannot alter a higher form of law. For example...because the supreme court has ruled that "flag burning" is protected under the 1st amendmant of the constitution...the only way this can be changed is by an amendment to the constitution. Congress can pass all the statutes it wants but they will all be voided by the supreme court until the constitution is changed.
I think your department of labor will be able to help you with your situation.
good luck.
