The cost of upgrading the West Coast Mainline, as an alternative to a high-speed London and Birmingham link, has risen by about 68%, the Department for Transport (DfT) has said.
In 2010 it was estimated an alternative rail package would cost about £5.3bn, but it has increased to about £8.9bn.
The DfT said prices for alternative schemes include the same allowance for cost overruns as the high-speed scheme.
No plans have been announced to take alternatives forward.
Some residents on the high-speed (HS2) route have already voice concerns during a public consultation, which ends in July, about the first stage of the scheme.
The government was likely to make a decision over the first stage towards the end of the year, the DfT said.
The alternative, the so-called Rail Package 2, incorporates the cost of upgrading the existing line to allow more trains with extra carriages, and was designed last year to test the case for HS2, the DfT said.
Rail Package 2 "would not be a viable alternative to a new high-speed rail network", it added.
The DfT said: "The West Coast Main Line is now twice [as] busy as when it was last upgraded - which caused 10 years of disruption.
"It provides far less new capacity than HS2 with the largest increases in capacity at off-peak times when they're less needed."
It said the intention was to have a public consultation in about 2014 over a proposed 'Y' shaped network with separate legs from the West Midlands to Manchester and Leeds.
Joe Rukin, one of the co-founders of Stop HS2, said: "It's unreasonable to compare the Rail Package 2 with the first stage of HS2.
"You've got to compare it with the full Y network, which is £32.2 billion, because Rail Package 2 delivers benefits across the country and not just this extremely narrow corridor that HS2 does."