SCP Foundation

SCP-2610-A, -B, -C, and -D were the siblings Simeon, Armond, Jorge, and Yvette Avakian

I think you should swap the order of Jorge and Yvette's names, as both photos of young women are referred to as -C.

sharply stunted mental faculties

Suggest something like "severely underdeveloped mental faculties", as "sharply stunted" doesn't quite feel in-tone.

Addendum 2610-1

All of the other addenda have a period before the designation (eg "Addendum 2610.2"). Also, some of the addenda have italicised notes explaining their provenance, but not this one - is this Foundation text, and the rest UIU? There wasn't much difference in the tone to distinguish them.

has, in experience, turned

I'm never sure with journal extracts whether the "errors" are deliberate or not, but I'll comment on them just in case. "with experience" would be the more common construction.

were I to find myself together with another conscious

"consciousness"

light shined through the cracks

"shone"

I wrote it nonetheless;

Suggest a colon at the end of the line.

suffering."

I don't think you need the closing quotation marks, as the journal excerpt doesn't open with one.

the vehicle was recovered and identified as belonging to A. Avakian, a dockhand from Boston whose fingerprints were recognized as also being from the previous scene

I see on a second read that you've laid the groundwork for this, but it didn't link up for me the first time through, especially having just read that no fingerprints from the first scene were recognized.

Perhaps something like: "the vehicle was recovered and identified as registered to A. Avakian through his employer, a shipping company in Boston. Mr Avakian's employment records included fingerprints, which were matched to some of the unidentified prints from the previous scene".

They're getting them on the line right now, Cooper

This transcript has references to "Cooper", but the relevant agent's name is given as "Agent Carter". Unless that's a nicknaming convention with which I'm unfamiliar, I suspect it might be an oversight.

Also, I wonder how important recognising -B is in the transcript. In a room of 400 naked people that would be hard enough, but with the distraction of the Colony's physiology, I struggle to believe that Agent Carter could pick -B in a line-up.

Agent Designation: Killroy

Is the double-l deliberate?

the assailants had boarded the train and were en route to the west coast

If the train was hijacked on the east coast, how did anyone know their final destination. "en route west" might be more realistic for the state of information at the time.

Attempts were made to halt the vehicle using explosives, but were similarly fruitless, as SCP-2610-E instances would quickly throw their bodies on top of any incendiary placed on the track

How would that stop the track itself from being damaged? And couldn't the track be destroyed miles in advance of the train's path? I wonder whether it's better for the UIU to decide early on (after the Kentucky incident) that it's safer to let the trains get through (to avoid civilian casualties from derailment and letting the Colony loose in a mid-west town) and set up a trap on the coast, once they intuit where the trains are headed. In fact, I wonder whether it's then better for the UIU to let the Colony get onto the boats, and have the F-111 bombers as their main plan, rather than a last-ditch idea.