BERLIN (AP) — Scheduled work to replace a part triggered a communications outage on Germany’s railway that brought all trains to a halt, stranding travelers across the country, the network operator said Wednesday as it faced criticism and questions over the embarrassing chaos.

The main railway operator, federal government-owned Deutsche Bahn, apologized for the abrupt halt in services late Tuesday, when all trains in Germany ground to a stop. Service resumed gradually about two hours later, after midnight.

Long lines formed at information desks as travelers tried to figure out how to reach their destination and where to spend the night.

Deutsche Bahn said it was offering taxi and hotel vouchers and, where possible, putting trains in place for would-be travelers to sit in while they waited. But passengers complained of a lack of information, hotel rooms were not available everywhere, and some travelers’ journeys stretched through the night.

The outage was the result of a problem with the GSM-R digital communication system used for internal communication on the railway network.

Deutsche Bahn said trains were running “largely seamlessly” on Wednesday morning, though there may still be some delays.

The head of the operator’s DB InfraGO infrastructure division, Philipp Nagl, said that the cause appeared to have been “the scheduled swap of a technical component.” He did not elaborate.

“We are analyzing with the highest priority how exactly this led to the fault,” Nagl said in a brief statement, adding that the company apologizes to its customers for the disruption.

The breakdown came after years of increasingly frequent complaints about train delays and service interruptions.

Deutsche Bahn is conducting thorough though disruptive overhauls of major routes after years of underinvestment in a bid to improve its performance, but any significant improvement is expected to take time.

The European Union’s most populous country has a railway network totaling some 33,400 kilometers (20,750 miles) in length, with 5,400 train stations and used by an average 50,000 trains per day. DB InfraGO says that makes it Europe’s biggest network.

“That all rail traffic in Germany comes to a halt because of a technical defect is a new low in already poor operating quality,” Oliver Krischer, the regional transport minister in North Rhine-Westphalia state, Germany’s most populous, told dpa.

He said there need to be “emergency mechanisms that prevent such a disaster in the future. People rely on reaching their destination at least somewhat punctually by rail.”