These tiers could categorize methods of operation in conjunction with safety improvements such as separation of passenger and freight traffic and advanced train control systems.
这些安全措施包括将客运流量与货运流量隔离，升级列车控制系统等。
2
With human supervisors in the passenger seat, seven test cars have driven 1, 000 miles without human intervention and more than 140, 000 miles with only occasional human control.
With human supervisors in the passenger seat, seven test cars have driven 1,000 miles without human intervention and more than 140,000 miles with only occasional human control.