“Did you see it, now?” he shouted.
Whyte stopped in his tracks, and the others with him.
“Did you see that train?” the policeman from the lower level asked again, as two more men came running up the stairs.
“What happened?” Wilson wanted to know.
“Didn’t you see it?” snapped Kennedy.
“Sure not,” the policeman replied. “It passed through up here.”
“It did not,” roared Whyte. “Down there!”
The six men with Whyte glowered at the three from the lower level. Tupelo walked to Whyte’s elbow. “The train can’t be seen, Mr. Whyte,” he said quietly.
Whyte looked down at him in utter disbelief. “You heard it yourself. It passed right below—”
“Can we go to the car, Mr. Whyte?” Tupelo asked. “I think we ought to talk a little.”
Whyte nodded dumbly, then turned to the policemen and the others who had been watching at the lower level. “You really didn’t see it?” he begged them.
“We heard it,” the policemen answered. “It passed up here, going that way, I think,” and he gestured with his thumb.
“Get back downstairs, Maloney,” one of the policemen with Whyte commanded. Maloney scratched his head, turned, and disappeared below. The two other men followed him. Tupelo led the original group to the car beside the station platform. They went in and took seats, silently. Then they all watched the mathematician and waited.
“You didn’t call me down here tonight just to tell me you’d found the missing train,” Tupelo began, looking at Whyte. “Has this sort of thing happened before?”
Whyte squirmed in his seat and exchanged glances with the chief engineer. “Not exactly like this,” he said, evasively, “but there have been some funny things.”
“Like what?” Tupelo snapped.
“Well, like the red lights. The watchers near Kendall found a red light at the same time we hit the one near South Station.”
“Go on.”
“Mr. Sweeney called me from Forest Hills at Park Street Under. He heard the train there just two minutes after we heard it at the Copley junction. Twenty-eight track miles away.”
“As a matter of fact, Dr. Tupelo,” Wilson broke in, “several dozen men have seen lights go red, or have heard the train, or both, inside of the last four hours. The thing acts as though it can be in several places at once.”
“It can,” Tupelo said,
“We keep getting reports of watchers seeing the thing,” the engineer added. “Well, not exactly seeing it, either, but everything except that. Sometimes at two or even three places, far apart, at the same time. It’s sure to be on the tracks. Maybe the cars are uncoupled.”
“Are you really sure it’s on the tracks, Mr. Kennedy?” Tupelo asked.
“Positive,” the engineer said. “The dynamometers at the power house show that it’s drawing power. It’s been drawing power all night. So at 3:30 we broke the circuits. Cut the power.”
“What happened?”
“Nothing,” Whyte answered. “Nothing at all. The power was off for twenty minutes. During that time, not one of the two hundred fifty men in the tunnels saw a red light or heard a train. But the power wasn’t on for five minutes before we had two reports again — one from Arlington, the other from Egleston.”
There was a long silence after Whyte finished speaking. In the tunnel below, one man could be heard calling something to another. Tupelo looked at his watch. The time was 5:20.
“In short, Dr. Tupelo,” the general manager finally said, “we are compelled to admit that there may be something in your theory.” The others nodded agreement.
“Thank you, gentlemen,” Tupelo said.
The physician cleared his throat. “Now about the passengers,” he began. “Have you any idea what—?”
“None,” Tupelo interrupted.
“What should we do, Dr. Tupelo?” the mayor’s representative asked.
“I don’t know. What can you do?”
“As I understand it from Mr. Whyte,” Wilson continued, “the train has… well, it has jumped into another dimension. It isn’t really on the System at all. It’s just gone. Is that right?”
“In a manner of speaking.”
“And this… er… peculiar behavior has resulted from certain mathematical properties associated with the new Boylston shuttle?”
“Correct.”
“And there is nothing we can do to bring the train back to… uh… this dimension?”
“I know of nothing.”
Wilson took the hit in his teeth. “In this case, gentlemen,” he said, “our course is clear. First, we must close off the new shuttle, so this fantastic thing can never happen again. Then, since the missing train is really gone, in spite of all these red lights and noises, we can resume normal operation of the System. At least there will be no danger of collision — which has worried you so much, Whyte. As for the missing train and the people on it—” He gestured them into infinity. “Do you agree, Dr. Tupelo?” he asked the mathematician.
Tupelo shook his head slowly. “Not entirely, Mr. Wilson,” he responded. “Now, please keep in mind that I don’t fully comprehend what has happened. It’s unfortunate that you won’t find anybody who can give a good explanation. The one man who might have done so is Professor Turnbull, of Tech, and he was on the train. But in any case, you will want to check my conclusions against those of some competent topologists. I can put you in touch with several.
“Now, with regard to the recovery of the missing train, I can say that I think this is not hopeless. There is a finite probability, as I see it, that the train will eventually pass from the nonspatial part of the network, which it now occupies, back to the spatial part. Since the nonspatial part is wholly inaccessible, there is unfortunately nothing we can do to bring about this transition, or even to predict when or how it will occur. But the possibility of the transition will vanish if the Boylston shuttle is taken out. It is just this section of track that gives the network its essential singularities. If the singularities are removed, the train can never reappear. Is this clear?”
It was not clear, of course, but the seven listening men nodded agreement. Tupelo continued.
“As for the continued operation of the System while the missing train is in the nonspatial part of the network, I can only give you the facts as I see them and leave to your judgment the difficult decision to be drawn from them. The transition back to the spatial part is unpredictable, as I have already told you. There is no way to know when it will occur, or where. In particular, there is a fifty percent probability that, if and when the train reappears, it will be running on the wrong track. Then there will be a collision, of course.”
The engineer asked: “To rule out this possibility, Dr. Tupelo, couldn’t we leave the Boylston shuttle open, but send no trains through it? Then, when the missing train reappears on the shuttle, it cannot meet another train.”
“That precaution would be ineffective, Mr. Kennedy,” Tupelo answered. “You see, the train can reappear anywhere on the System. It is true that the System owes its topological complexity to the new shuttle. But, with the shuttle in the System, it is now the whole System that possesses infinite connectivity. In other words, the relevant topological property is a property derived from the shuttle, but belonging to the whole System. Remember that the train made its first transition at a point between Park and Kendall, more than three miles away from the shuttle.
“There is one question more you will want answered. If you decide to go on operating the System, with the Boylston shuttle left in until the train reappears, can this happen again, to another train? I am not certain of the answer, but I think it is: No. I believe an exclusion principle operates here, such that only one train at a time can occupy the nonspatial network.”