I believe your posts stated earlier (as CN Sparky said: very eloquent) you were inquirying about the CN. Well if you fill out the entire online application you get the following message at the completion of the application:
I understand that I must be legally entitled to work in the U.S.; my employment is contingent upon my passing a pre-employment physical which
will include drug testing; that my employment will begin with a period of probation; and that there are regulations and policies that apply to employment at CN.
In the U.S., CN complies with firmly established rules and regulations governing drug and alcohol testing after accidents or incidents and the random testing of employees in key groups.
http://www.cn.ca/en/corporate-citizensh ... llness.htm
While you may not be in the "key groups" you may still be included if an incident happens.