You can stop it at the border.

Orlov:

[enters the trailer, sees Bond in Mischka's 'Twin One' clothes from behind and mistakes him for Mischka]
Leave that! Let's go. James Bond:
[turns around, facing Orlov with pointed gun]
Let's stay. Sit on that box. Hands on your knees. Come on, move! Now why is that bomb on the train? Orlov:
Who are you? James Bond:
I'm British Secret Service. Orlov:
You should be more concerned about getting out of here alive. James Bond:
I am more concerned about an atomic bomb exploding on a US Air Force base! You surely can't be inviting a full-scale nuclear war? What happens when the US retaliates? Orlov:
[grinning]
Against whom? James Bond:
[frowning, realizing Orlov's scheme]
My God... of course. Our early-warning system will rule out the possibility of that bomb having been launched from Russia or anywhere else. Everyone will assume incorrectly... that it was a American bomb triggered accidentally. Orlov:
Well, that would be the most plausible explanation. James Bond:
Europe will insist on unilateral disarmament, leaving every border undefended for you to walk across at will! And it doesn't matter a damn to you I suppose that thousands of innocent people will be killed in that "accident" of yours? Orlov:
Better than letting a handful of old men in Moscow bargain away our advantage in disarmament talks! James Bond:
On your feet, General. You're going to stop that train. Orlov:
[they hear hoot as the train starts to move off]
It's too late. James Bond:
You can stop it at the border. [suddenly the door opens. Bond turns around to see a Russian soldier. Bond kills the soldier before he manages to point his rifle, and meanwhile Orlov escapes]