Final answer:
The worst problem caused by early surface rail systems was safety concerns and accidents. Populous cities like New York experienced frequent accidents and trolleys went underground to address crowded streets.
Step-by-step explanation:
The worst problem caused by early surface rail systems was safety concerns and accidents. Populous cities like New York experienced frequent accidents, as depicted in an illustration from 1895. To address crowded streets, trolleys eventually went underground, leading to the construction of subway tunnels.