Section 4. An employee of a railroad corporation injured by any locomotive, car or train used contrary to any provision of sections one hundred and fifty-five and one hundred and fifty-seven to one hundred and fifty-nine, inclusive, of chapter one hundred and sixty shall not be deemed to have assumed the risk of such injury, although he continues in the employment of such corporation after the unlawful use of such locomotive, car or train has been brought to his knowledge. An employee of a railroad corporation injured by any locomotive, car or train by reason of the negligence of any other employee of the corporation shall not be deemed to have assumed the risk of such injury.