An advisory board to the Pentagon is fueling new debate on mobile nuclear weapons by saying at least some MX missiles should travel on their rail-car launchers at all times if that mode of basing is adopted.

The Air Force previously has sought to ease public concerns about nuclear weapons on the rails by saying such missiles would leave their armed garrisons only in times of crisis.But the influential Defense Science Board said that plan would be more likely to provoke the Soviets or any other adversary.

"The problem with keeping them in the garrison at all times is that a president could raise tensions higher simply by sending the trains out - and wouldn't be prepared to take that risk," one source said Thursday.

"That doesn't help your nuclear deterrent any. But if you have at least a few out there all the time, the Soviets know you're serious and that you've got a mobile missile that they can't strike first."

The review by the science board was first reported Thursday by the Los Angeles Times. The Defense Department declined public comment on the matter.

The Bush administration - like the Reagan administration before it - is struggling to develop a plan for adding a mobile nuclear missile to America's arsenal. The idea is to make it more difficult for the Soviet Union to target U.S. forces and thus deter Russia from ever considering a first-strike attack.

The Air Force and Pentagon have told Congress they would like to buy 50 more huge MX missiles and place them on rail-car launchers. Congressional Democrats, however, would prefer to see the development of a small, one-warhead Midget-man missile that could be deployed on a truck launcher.

Proponents and critics alike agree that placing 50 new MX missiles on rail cars would be vastly less expensive, about $12 billion, compared with almost $25 billion for 500 Midgetman missiles. But there is also agreement that a Midgetman force would probably be more survivable against attack.

That is because the Air Force, in pushing for mobile MX missiles, has always stressed it would mollify public safety concerns by keeping the missiles inside armed garrisons - ready for dispersal on the regular rail network only at times of high tension.

It is that assumption that the science board is preparing to challenge, said one source.