Murder on the Omega Express

The train chugged along the tracks morosely. Tunnel after tunnel, it pushed on its merry way to Omegalopolis. Ross Campbell, Zero Caspian and the titular Crimebot sat in cabin 3C. Ross looked out the window at the passing countryside.

"Sigh," he said out loud, "The countryside here is beautiful. I wish we weren't constantly in danger," he finished. Just then, a gunshot rang out!
"What was that?" exclaimed Crimebot.
"A gunshot," replied Ross.
"Oh, right," said Crimebot.

A half hour later, Crimebot burst awake. "WAIT A DAMN MINUTE," he ejaculated, "Guns aren't allowed on the plane!"
"We're on a train," Ross said groggily.
"Oh, right," said Crimebot.

Another half an hour later, Crimebot burst awake again. "WAIT A DAMN MINUTE," he blurted out all over the cabin, "Guns aren't allowed on the train!"
"Wait, you're exactly right!" said Ross, half-awake, "But what does that have to do with anything?"
"Silly human," said Crimebot, "Your memory is too unprecise!"
"Imprecise?" said Ross.
"UNPRECISE, SILENCE HUMAN!" Crimebot bellowed. "Anyways, an hour ago a shot was fired on the train!"
"Oh right," exclaimed Ross. "We should investigate!"
"What about Zero Caspian?" asked Crimebot.
"Let him sleep, he's had a rough night in those criminal prisons," Ross replied.
"IRONY!" yelled Crimebot.
ey
Ross and Crimebot burst into cabin after cabin. Finally, after waking up a dozen sleeping passengers and interrupting a rather spirited bout of coitous, Ross and Crimebot found a corpse. "Gasp!" exclaimed Ross.
"Aeiiiiiii!" exclaimed Crimebot. Then Crimebot yelled out for the train constable.

About fifteen minutes later, the train constable arrived. He was an enormously fat man of late years who was constantly sweating and having to dry himself with a handkerchief. He spoke slowly with a Southern drawl. "Now, now what I say is going on here?" he asked Crimebot. "What are you two doing a-hovering over this here corpse?"
Crimebot stepped forward and slapped the constable across the face. "Can't you see I'm a police officer?" The constable looked stunned for a moment, but then came to his senses and quickly apologised to Crimebot for suspecting him.
"What's happened here, my friend? the constable asked, finally.
"About an hour ago, a shot rang out. We looked around and found this man, dead as a doornail," explained Ross.
"Well, well now what happened to him?" asked the constable."
"HE'S BEEN SHOT YOU IDIOT!" yelled Crimebot excitedly.

Moments later, Crimebot walked over and opened the window. "Come here and look at this," he said to the constable. As the constable walked over, Crimebot took a step back, and then jumped forward with all his might, pushing the constable out the window into the ravine the train was passing by. As the fat constable went screaming to his death, Ross leapt over and asked Crimebot what the hell he was thinking.
"Who's the one man with a gun on board this train?" Crimebot asked of Ross.
"Me?" came Ross' reply.
"Other than you!" said Crimebot.
"Who?" Ross asked.
"The constable!" Crimebot explained finally.
"Train constables don't carry guns, Crimebot," said Ross, shaking his head.
Crimebot was silent for a second. "He was pretty stupid though, anyways," he said at long last.
"Well, it looks like it's up to us to solve this mystery, eh?" said Ross.
"Yeah, let's examine this room for clues!" said Crimebot, who got busy examining the room for clues.

Little did Crimebot and Ross know that while they examined the cabin for clues, a sinister man was plotting to kill his next two victims, a certain ex-marine and a handsome robot...