Leading senators have warned the White House that George Shultz's new intermediate-range missile (INF) treaty faces big trouble unless President Reagan pledges the $1 billion or more needed to complete a new spy-in-the-sky satellite to verify Soviet compliance.

Giving teeth to the Senate warning is a confidential CIA analysis showing for the first time that up to several hundred long-range SS-11 missiles, presumed obsolete and believed destroyed, may in fact have become rail-mobile. Discovered fortuitously by an aging ''image'' satellite, the SS-11s in their new role are concealed on special missile trains parked on rail spurs, three launchers per train.

There is no proof that the new ''image'' satellite these Senate leaders privately demand can prevent all Soviet cheating on the INF treaty, under which 441 Soviet SS-20 mobile missile launchers are supposed to be destroyed. But it would help. Lawmakers aware of the CIA's shocking discovery about the SS-11s as a minimum demand President Reagan's pledge to allocate all secret funds needed for the new satellite.

First disclosure that the United States is developing a new super-picture ''image'' satellite came in Bob Woodward's well-documented new book, ''Veil.'' The late William Casey, Woodward writes, as CIA director ''faced a huge decision on one of the U.S. intelligence community's most top-secret and important . . . projects in overhead systems . . . billed as the biggest technological spy development of the 1980s.'' The satellite would provide all-weather, day-and-night capability.

But one senator told us that, given the thin gruel of the federal budget, ''the administration is waffling'' on allocating money needed to develop the new satellite. This senator, a moderate Democrat who is leading the campaign with the White House, told us: ''Ratification of the INF must be tied to a pledge by the president to enhance and upgrade our satellite imagery to give us at least a good shot at verification.''

The main reason for heightened distrust is the CIA's tightly held evidence of Soviet handling of the old SS-11 intercontinental-range missiles. The story starts with the December 1976 launch of the now-aging U.S. KH-11 image satellite, the first to provide instantaneous transmission of pictures to intelligence analysts. No longer were pictures dropped in canisters from the satellite.

When the Soviets saw no canister drops, they first assumed the KH-11 was a ''signals'' satellite to collect intelligence by monitoring Soviet communications. Thus they were thrown off guard, and worked openly on missile operations. That briefly gave the United States what the CIA calls ''unexpected intelligence.'' Part of that rich picking was the clear picture of an SS-11 missile erected on the soft-pad site of an SS-4 medium-range missile.

The picture was proof the SS-11 had been given the hitherto unsuspected role of a mobile missile, not limited to being fired from a hardened silo. U.S. intelligence analysts were surprised, but could learn little more. The Soviets, soon realizing the KH-11 orbiting over their territory was taking pictures rather than monitoring messages, quickly concealed other juicy scenes during the satellite's regular overflights.

Intelligence analysts filed away the few pictures they had. What next happened shocked the Analytical Division of the CIA's Soviet Affairs Directorate: an unmoving image transmitted this summer over a period of several weeks by the KH-11 of a 12-car train on a rail spur in Siberia. Other satellites monitored the train in direct communication with the headquarters of Soviet Strategic Rocket Forces.

CIA officials, ordering an immediate analysis, came to an inescapable answer: this was a missile-launch train carrying what the communications intercept strongly indicated were SS-11 missiles. Photo interpreters then recalled past pictures, puzzling to them for years, of 12-car trains complete with engines standing apparently aimlessly on rail spurs. A check of relevant KH-11 imagery back to 1976 turned up ''several score'' pictures showing stationary trains that were occasionally moved from one rail spur to another.

The CIA said ''no comment'' when we asked about these train-mobile SS-11s. But inside the agency, they are known as ''the stockpiled reserve, covert softlaunch ICBM problem.''

Senate leaders privately discussing the impact of the SS-11 revelation on INF ratification predict President Reagan will have no problem with liberals in appropriating money. Liberals will support spy satellite money because they want the INF treaty so badly they will do almost anything to get it.

But that may not be the end of the INF treaty's trouble. Even if the $1 billion for the spy satellite is forthcoming, the full story of the presumed ''obsolete'' SS-11s and what it means for verification could build serious opposition to ratification of the INF agreement.