When Casey is on top of the train car looking at the terrorists through a window on the edge of the roof, Travis Dane says "300,000 pages of code. Or 60 minutes of triple-X rubber-and-leather bondage porno.", his mouth is clearly not moving.
When the train is stopped so the mercenaries can retrieve the CD, after the train is shown slowly reversing backwards, we see Dane talking to Penn. Behind Dane there is a window showing the train is still moving, and fast.
When he shoots down the Stealths, Grazer 1 HAS to be somewhere between Denver and LA. It is directly above the train. There are no satellites on that path. The satellite path for Delta comes down through Canada to Washington. and nowhere near the train's path.
The space shuttle seen launching in the opening sequence can be identified as Space Shuttle Columbia (OV-102) due to the presence of the "SILTS" pod on the tip of the tail (visible as a bump on the tip of the tail) which was only placed on Columbia. When the shuttle is seen in orbit, it is now labeled as Space Shuttle Discovery (OV-103) and the SILTS pod is missing.
When Ryback is hanging from/standing on the cliff after the train stops, a close-up shot shows both of his feet together on one small ledge. A long shot about 10 seconds later shows his feet on separate rocks.
When Penn slips the noose around Sarah's neck, it's tied off with a knot. In the very next shot the end is crimped on with a metal clasp, no knot visible. It changes back and forth several times after that.
In the ATAC control room before the two captains enter their passwords to gain control of the satellite, the screen with the asterixed-out passwords which appears after the transfer is completed is visible.
The spotlights behind Travis Dane during the "Paris Feed" disappear when he is on screen at the command center. However, by the staging of the spot lights on the back drop on the train, the lights should be visible to those in the command room.
When Dane leaps onto the helicopter's ladder below Ryback, he's holding on with his hands while most of his body is dangling from it, but in the immediate next shot he is standing entirely on the ladder, well away from the bottom.
When the 20-minute countdown to the firing of Grazer I at the Pentagon begins, the military brass watch as the countdown clock ticks down from 20:00, with a beep sounding at each passing second. When the clock hits 19:54 the scene shifts to conversation among the officers while the beeps continue to be heard. A careful count of the beeps indicates that when the clock is next shown it should read 19:41, but after the passage of 13 seconds, when the clock is again seen it reads 19:50.
When ATAC chooses to fire their final Pegasus missile, there are still 10 ghost satellites remaining, plus Grazer's actual orbit. Of these 11, Gamma is the only satellite above US soil. It turns out not to be Gamma.
The train is travelling West to LA. To do that, the satellite must come from the West and head east to attack Washington. When the satellite is revealed to be Delta, Delta is travelling the opposite direction.
When Travis Dane is "posing" to be in France, the live feed is stopped and the screen goes black. A shot of the crowd watching the now-black screen shows a window which is reflecting an image of the screen with Travis Dane still on it.
After recovering the CD from under the train, Penn says to Sarah, "Guess your uncle wasn't that good after all." But none of the bad guys has yet established the exact connection between Sarah and Casey, except that Sarah is traveling as Casey's '+1' on the passenger manifest.
When Dane walks up to the door of the sleeping car that Linda and David were having sex in right before the train was taken over. He looks in and gives his line "Well, here you are. The last place anyone expected to find you." When the formally love making couple are shown Linda's hair is neat looking with her hair all in place but after the camera cuts back to her a second time after Dane says "Amazing." her hair is messed up with several locks of hair covering her left eye. It would have made more since for the two camera cuts to be switched around since her hair should have been messed up first and then she could have off camera for a brief second that she brushed it out of her eye. Instead it makes it look like she messed her hair up on purpose. What was she trying to do? Hide her eye?
While Ryback is hanging on the rope, the porter takes a few pot shots at the terrorists. Prior to shooting he says, "Safety off". Glock handguns do not have a safety that can be switched on or off. It is integrated into the trigger assembly and turns 'off' as the trigger is depressed.
When Ryback is hanging in the ravine and the rope is shot loose from the train he is still climbing. Physics don't work this way, once the rope was loose, him pulling on it would've only dragged it over the edge of the ravine faster.
In order to defend herself, Sarah Ryback sprays Marcus Penn with a non-lethal spray, and says "[This is] Mace!" The spray has little to no effect on Penn, who then responds by saying that it is "Not mace, sweetheart. Pepper spray. Sold to civilians. But once you get used to it, it just clears the sinuses." Pepper spray is actually much more effective than the classic Mace (brand name). Penn, as a highly trained mercenary, should have known the difference.
While a geostationary satellite like the Ares-1 could in fact reach a maximum orbital velocity that would allow it to easily strike all of the targets Dane selects throughout the movie (starting in China, then moving to the Middle East, then arriving over North America and taking out the two Stealth bombers before taking aim at the Pentagon), the last two items are not plausible as presented, because Dane found out they were there (and where they were) at a point where it would have still taken Ares-1 longer than its max burn rate to get there and destroy the Stealths. In this situation, the train would have been destroyed in an air strike.
When the shuttle releases the satellite the PBDs (payload bay doors) are partially closed. Shortly after achieving orbit the PBDs are fully open to release built-up heat in the cargo bay, and the doors are generally not closed before re-entry, as well as being fully open during deployment/retrieval of satellites.
The actual credits for this movie show that Dale Dye played Col. Darza. This is incorrect in more than one way. Not only does Admiral Bates call him Captain Garza in the movie but his character is a continuation from the first Under Siege. On a more factual basis, Colonel is not a naval rank and Captain Garza wears a Naval uniform.
Several times Ryback opens panels on the outside of the train. When they are hinged towards the front of the train, the air rushing past should blow them back against the side of the train; if the hinges are at the back then the panel should fly open. Neither of these actions appear to happen. Only much of the train footage was shot live on location and the real world physics prove that it is possible,
When the CD was stolen from the computer on the train, it is not fair to assume that it would disrupt the program to the point where it would reveal Grazer's location to the team at ATAC since it was just a CD of targeting codes.
After Dane receives the passwords from the agents, he then enters them onto one computer using two keyboards. This would actually work since this is how it operates at Grazer Control and his equipment on the train are duplicates, or upgrades, of the Grazer computers.
When they take over the train, Ryback beats up one of the mercenaries, either killing him or knocking him unconscious, and throws him from the train. Another mercenary says over the radio they have a casualty. Despite this nobody seems concerned or suspicious about what happened to this mercenary, and sensed there may be someone aboard the train trying to thwart their plans. However, the car is full of cooks. The assumption of the mercs was that one or all of them overpowered him then threw him out. When they shot up the car they found plenty of dead bodies and assumed they were the culprits since that's who they found. They had no reason to assume there was anyone else involved.
In the first movie - 1992 - Casey Ryback was a Chief Petty Officer (CPO) (E-7). In this movie, 3 years later, he's a retired Lieutenant (O-3). While it is rather common for commissioned officers to rise from the enlisted ranks, such officers are usually no higher than E-5 before they are commissioned. CPO's are enlisted men with a great deal of responsibility and authority - often more so than a junior officer - so it's uncommon for them to even go for a commission. Even so, it's highly unlikely that anyone (CPO or otherwise) would accept a commission and then almost immediately retire as seems to be the case here. Finally, it usually takes four years from the time an officer is commissioned at Ensign (O-1) until he is promoted through Lieutenant, Junior Grade (O-2), and then Lieutenant.
The scenario in the movie would make sense only if (1) More than 3 years elapsed between the first and second films (which is never defined either way), and (2) if Ryback had been a RESERVIST rather than RETIRED (and he is explicitly said to be retired).
Shortly after attempting to send the fax from the payphone, Casey is seen pulling a dead mercenary through a door by his arm, shortly before another mercenary comes down the stairs. Casey grabs the dead guy's left leg to pull it, but both legs move together as the actor moves them himself.
In the final fight Seagal and the mercenary roll down the stairs fighting. You can only see the back of his head but this clearly isn't Seagal. The hair of the stunt-double is dim and short, not shiny and combed like Seagal.
When Ryback is off the train and runs to the truck to start it, he just pops the hood open from the front. For a 1981 Chevy, this is not possible. You have to pull the hood release from inside, then open the latch from the outside.