Exit 17

Introduction

Gordon, Alyx and Barney need to escort the last groups of citizens to the escape trains before leaving City 17 for good.

Maps

ep1_c17_05 - the parking lot and train repair.
ep1_c17_06 - the train loading area and outro ride.

Architecture

The trainstation from which the escape trains are leaving the city has more to do with servicing the trains and loading cargo as opposed to the intro trainstation in Half-Life 2. As such, it features more industrial choice of textures and props, less fancy looking. The surrounding buildings use metal and brick materials and look utilitarian. The architecture here has more in common with Route Kanal and Water Hazard.

During the outro ride, the train passes by some overpass bridges, water towers and highrises, again reminiscent of the canals sections of Half-Life 2.

Afterward, it transitions to a part of the track framed by rocky slopes and pine trees, foreshadowing the natural environment of Episode Two.

Environment

Much like in the previous chapter, ash is drifting through the air, the sky is tinted in yellow, brown and reddish, the ambient lighting is warm. In a few key points, fluorescent lamps and Combine lights provide contrasting cold color to help guide the player.

The environment is relatively intact, if cluttered by car wrecks, shipping containers and industrial-themed props. The buildings are mostly undamaged and have electrical power. There's even some green grass textures on the slopes bordering the parking lot.

NPC's

Numerous Combine Soldiers arrive from the hospital by rappelling and jumping from the balconies. Others enter the train repair area through the windows and later, broken doors. In the second level, soldiers accompany the Strider.

Several Minelayers show up to drop Hoppers on the parking lot. A couple City Scanners fly above the escape trains and are engaged by the rebels (inaccessible to the player).

A Combine APC, a Combine Sniper and Metrocops appear during the escort.

Citizens are featured prominently in the first level, as escorting them is the point of that level.

A Strider (or multiple; see the trivia) serves as the final boss in the second level.

Poison Headcrabs make a minor appearance during the Strider fight.

A Combine Dropship is seen flying above the parking lot, but that's all it does.

An Advisor is seen once again on the monitor at the beginning of the chapter, interrupting Dr. Kleiner's recorded broadcast.

A scripted Combine Gunship appears during the outro ride.

Trivia

Up until at least 2013, there existed a sixth 'chapter', titled Coming Soon, which served to show a teaser[1] of Half-Life 2: Episode Two. It was accessible either from the main menu or triggered after Exit 17's end credits. Valve scrapped this video (as well as Episode Two's intro recap video of Episode One's events) in one of the updates.
The strider in ep1_c17_06 can be defeated without getting around the arena and picking up RPG ammo; it requires stocking up on missiles and grenades, utilizing explosive props and/or playing on Easy difficulty. However, this is futile, as the strider's spawning template will be triggered again upon its death, effectively making it infinitely respawning.
The back of the hospital in ep1_c17_05 differs a lot from both its facade and interior structure; in fact, it doesn't have the same amount of floors, and features spacious balconies, which would be unusual in a hospital. Given that the hospital levels are numbered as ep1_c17_02a/b, and Exit 17 starts on ep1_c17_05, this may be a result of 'stitching' levels together late in production.
Like in Point Insertion, a Nova Prospekt sign can be seen attached to the fence near the tracks in ep1_c17_06.