Glossary – No O'Hare Flyer

The O’Hare Flyer debate is full of railroad and regulatory terms. Here is what they mean, in plain language, and why each one matters for this project.

Jump to: Airport Transit System (ATS) · Altenheim Subdivision · At grade · B-12 · Battery-electric multiple unit (BEMU) · Berm (embankment) · Class I / Class III railroad · CMAP · Common carrier · Dispatching · Double track / single track · Dwell time · EcoCAT / Part 1075 consultation · Eisenhower trench · FAA Part 77 · Flyover · Forest Preserve District of Cook County · FRA · Grade crossing · Grade separation · Headway · Illinois Commerce Commission (ICC) · Illinois Natural Areas Inventory (INAI) · Interlocking · Letter of intent (LOI) · Metra North Central Service (NCS) · NEPA · NITA · Passing siding · Preemption · Quad gates (four-quadrant gates) · Rail carrier · Ridership study · Right-of-way (ROW) · Series LLC · Surface Transportation Board (STB) · Third rail · Trackage rights · Waukesha Subdivision

Airport Transit System (ATS): O’Hare’s free people-mover connecting the terminals, the parking garages, and the Multi-Modal Facility, where Metra’s O’Hare Transfer station is.
Altenheim Subdivision: CSX’s freight line in the Eisenhower Expressway trench between Chicago’s West Side and Forest Park, once the Baltimore & Ohio Chicago Terminal. The Flyer would use it to reach the Eisenhower corridor. It is the only segment where the sponsor has any railroad agreement (a nonbinding letter of intent).
At grade: At street level. A railroad “at grade” crosses streets with gates and signals rather than bridges. The opposite is grade-separated.
B-12: The junction in Franklin Park where the Canadian National line meets Metra’s Milwaukee District West line (owned by CPKC). Metra’s North Central trains join the CN line here. The Flyer would need a flyover at B-12 to cross without conflicting with Metra.
Battery-electric multiple unit (BEMU): A passenger train with motors and batteries built into the cars themselves, with no separate locomotive. It charges at the ends of the line. The Flyer proposes BEMUs; Metra has ordered similar trains from Stadler for the Rock Island line.
Berm (embankment): A raised bank of earth that carries the track above street level. The sponsor proposes a 15-foot berm through River Forest, topped by a 54-inch sound wall and an 8-foot fence — about 27 feet in all.
Class I / Class III railroad: Federal size categories. Class I railroads are the giants — Union Pacific, CSX, Canadian National, BNSF, CPKC, Norfolk Southern. Class III railroads are short lines, often serving a handful of customers. The Flyer says it has created a Class III railroad, “Great Western Railroad,” to carry a few freight cars at night. Carrying freight for hire makes it a rail carrier under federal law, which brings federal preemption.
CMAP: The Chicago Metropolitan Agency for Planning, the region’s official planning body. Its long-range plan, ON TO 2050, left an O’Hare express train out in 2018–19 on ridership and equity grounds.
Common carrier: A railroad that must carry freight for anyone who asks, at published rates. A Class III common carrier is a rail carrier in the eyes of federal law.
Dispatching: The control of which train may occupy which track and when. On shared or single track, the dispatcher decides who waits. The Flyer’s plan to use the freight railroads’ tracks “in an emergency” depends on those railroads’ dispatchers agreeing — which requires an agreement that does not exist.
Double track / single track: Two tracks let trains pass in opposite directions anywhere. One track means trains in opposite directions can pass only at a passing siding. The Flyer is single track for roughly 14 of its 20 miles.
Dwell time: The time a train sits at a station with its doors open. Every added stop adds dwell plus braking and acceleration, which is why an express train resists added stops.
EcoCAT / Part 1075 consultation: Illinois’ required review, run by the Department of Natural Resources, before a state agency or local government approves anything that could harm a protected species or a Natural Areas Inventory site. Thatcher Woods Prairie is INAI site No. 1431, so approvals affecting it must go through this consultation first.
Eisenhower trench: The below-grade cut carrying I-290 west from downtown. CSX’s Altenheim tracks run along it. The sponsor proposes 110 mph here.
FAA Part 77: Federal aviation rules for structures near airports. A 90-foot structure over O’Hare taxiways requires FAA review of its effect on aircraft operations, plus a change to the airport’s layout plan and federal environmental review.
Flyover: A bridge that carries one track over another so trains on the two lines never cross at the same level. The Flyer would need flyovers at B-12 (over Metra/CPKC), over the B&OCT near the Near West Side, and a 90-foot structure over the O’Hare taxiways. Each one is a major construction project.
Forest Preserve District of Cook County: The public body that owns Thatcher Woods. By state law it has had no authority to sell its land since 1923; its land cannot be condemned; and by policy it grants easements only to public transportation agencies — not private companies.
FRA: The Federal Railroad Administration, which sets safety rules for railroads, including standards for passenger trains and track speeds.
Grade crossing: A place where a road crosses railroad tracks at the same level, protected by gates, lights, and bells. The sponsor counts nine grade crossings between Madison Street and O’Hare: four in River Forest (Chicago, Iowa, Augusta, Thatcher), Madison Street, and four north of Thatcher Woods (1st Avenue, 5th Avenue, George Street, Belmont Avenue), plus Chestnut Street, which it proposes to close.
Grade separation: Replacing a grade crossing with a bridge or underpass so road and rail never meet. Expensive, and in Illinois it requires an order from the Illinois Commerce Commission, which also decides who pays. The sponsor proposes grade separations in River Forest and at Madison Street; the four crossings north of Thatcher Woods may remain at grade with quad gates when service begins.
Headway: The time between trains. A 15-minute headway means a train every 15 minutes in each direction — one every 7½ minutes past any given point.
Illinois Commerce Commission (ICC): The state agency with sole authority over railroad grade crossings in Illinois: whether a crossing may be built, closed, or separated, how, and who pays. Every crossing change on the Flyer route needs an ICC order. It is one of the few venues federal law leaves fully in state hands.
Illinois Natural Areas Inventory (INAI): The state’s official register of the highest-quality remaining natural land. Thatcher Woods Prairie is site No. 1431, Category II (habitat for state-listed species). INAI listing triggers the EcoCAT consultation.
Interlocking: A junction where tracks cross or converge, controlled by signals so only one train can use it at a time. A-2 (Western and Kinzie) and B-12 (Franklin Park) are the two that limit Metra’s ability to add O’Hare service.
Letter of intent (LOI): A written statement that two parties intend to negotiate an agreement. It is not a contract and binds no one. CSX’s letter of intent is the only railroad document the sponsor has.
Metra North Central Service (NCS): Metra’s line from Union Station to Antioch, which stops at O’Hare Transfer next to the airport’s Multi-Modal Facility. About 34 minutes from downtown today, with limited service because Canadian National owns the track north of Franklin Park and will not add trains without capacity improvements. It is one of the two public alternatives to the Flyer.
NEPA: The National Environmental Policy Act, which requires federal agencies to study environmental impacts before approving a project. It applies to the Flyer wherever a federal approval is needed — the FAA for the airport structure, the Surface Transportation Board if it takes jurisdiction over construction.
NITA: The Northern Illinois Transit Authority, which replaced the RTA on September 1, 2026 and now controls regional fares, service standards, and capital spending for CTA, Metra, and Pace.
Passing siding: A short stretch of second track beside a single-track line where one train waits for another to pass. The Flyer’s six miles of double track are, in effect, one long passing siding; every meet between trains must happen there.
Preemption: The legal rule that federal law overrides state and local law in an area Congress has claimed. For railroads, the Interstate Commerce Commission Termination Act (ICCTA) gives the federal Surface Transportation Board exclusive authority over “rail carriers,” so village zoning and nuisance rules generally cannot stop a railroad’s operations. This is why the Flyer wants to be a rail carrier, and why the campaign focuses on the ICC, the FAA, the STB, and the City Council rather than on local ordinances.
Quad gates (four-quadrant gates): Crossing gates that block both lanes on both sides of a crossing, so cars cannot drive around a lowered gate. Safer than standard two-gate crossings, but still a crossing at street level with trains every 15 minutes at speed.
Rail carrier: The federal term for a railroad that provides common-carrier transportation for compensation. Rail carriers get federal preemption; passenger-only operators that do not connect to the interstate network may not. This is the distinction the Flyer’s Class III subsidiary is designed to settle in its favor.
Ridership study: A forecast of how many people will ride and pay. The sponsor says its first study exceeded its projections and a second is coming; neither has been released. The only public data are Metra’s 2024 pilot (about 120 riders a day) and CMAP’s 2019 analysis.
Right-of-way (ROW): The strip of land a railroad owns or controls for its tracks — typically 50 to 100 feet wide. The Flyer proposes to build its own separate track inside the freight railroads’ rights-of-way, which requires their agreement.
Series LLC: An Illinois limited liability company that can create internal “series,” each with its own assets and liabilities walled off from the others. O’Hare Flyer LLC is organized as a series LLC. Which series would sign contracts with the City is a fair question.
Surface Transportation Board (STB): The federal agency that regulates railroad mergers, line construction, and the rights of rail carriers. It is reviewing the Union Pacific–Norfolk Southern merger (Docket FD 36873) and would review any filing by the Flyer’s Class III subsidiary.
Third rail: A powered rail beside the running rails that feeds electricity to a train. The Flyer would use third rail only at the two terminals and a charging track, running on batteries for the 18 miles between.
Trackage rights: An agreement letting one railroad run its trains over another’s tracks. Metra runs its North Central trains on CN’s line under such an agreement; CN controls how many.
Waukesha Subdivision: Canadian National’s name for the line through River Forest, Thatcher Woods, and Franklin Park — the former Chicago & Wisconsin / Soo Line route built in 1885–86. It is dormant south of Franklin Park.