"Right now it's a matter of trying to determine the demand and trying to mitigate the cost," said VRE spokesman Mark Roeber.
For about $50,000, the commuter train service initially will add access to the two rear cars of each train. Currently most trains are about six cars long, meaning that as much as half the train would be covered if the signal carries to the third car, Roeber said.
But eventually the commuter agency hopes to run 10-car trains when new, more powerful locomotives arrive, which would leave many riders without the Wi-Fi perk.
Already the "quiet cars" -- one car per train designated for riders who would like to sleep or work undisturbed -- fill up quickly. But Roeber said he doesn't think riders will crowd into the Wi-Fi cars immediately.
"A lot of people get on the train and don't do anything. They're relaxing, decompressing," he said. "I don't foresee it as being an issue."
Riders have been asking for the service, especially as consolation when delays and breakdowns make them late to work.
The agency waited to add any wireless service until it could be sure that each train line could have a full signal the entire way, Roeber said. Officials didn't want riders to have the expectation of Internet service then hit a dead spot en route. "We would look bad," he said.
Cell phone companies have fleshed out their networks, and Verizon has shown it can provide continuous service, he said.
VRE is the latest transportation service pledging to offer the perk as smart phones proliferate. Washington Dulles International and Ronald Reagan Washington National airports plan to start offering the service for free this spring. Amtrak announced this summer that it plans to expand its Wi-Fi service, while intercity bus companies such as Greyhound's Bolt Bus have been using it as a selling point to attract riders.
Meanwhile, Metro is expanding its cellphone service as part of a deal with Congress. It must have service in all its tunnels and stations by fall 2012. Officials have said the $40 million contract with national carriers Sprint Nextel, AT&T, T-Mobile and Verizon Wireless eventually will give the system wireless capabilities.