Most stations are of a modular design dating back to the initial system, extended and improved with two side platforms, each with separate access from the street, and platform canopies, although few examples remain of the original, distinctive rounded roof design. Stations are unstaffed, except the underground stations at , Stratford International and Woolwich Arsenal for safety reasons, a few of the busier interchange stations, and City Airport, which has a ticket office for passengers unfamiliar with the system. Canning Town, Custom House and Prince Regent are normally staffed on the platform whenever there is a significant exhibition at the ExCeL exhibition centre.