Q. He leaves his practice and works the emergency 58 miles away. What can he deduct as mileage?
A. None of his mileage is deductible. That assumes his emergency room job is as a regular W-2 employee.
Despite the May 31, 2019 date on the original question and responses, the above answers actually apply to 2017 and earlier tax returns. The tax law changed in 2018 and employees my no longer deduct job expenses, including mileage between jobs.
If the doctor is working the ER job as a self employed contractor, the 58 miles , between the two jobs is deductible on his Schedule C. This is true whether his regular (practice) job is as an employee or a self employed person. The mileage from the ER back to home is not deductible. The mileage from his home to his practice and the mileage back home from his last work place, of the day, is considered commuting, and is not deductible. Only the mileage between the two jobs sites is deductible.
