A block control system for your 3-rail, AC powered toy trains, allows multiple trains to run automatically on your "mainline" track.

Other "industrial" or "yard" layout areas are operator controlled. Trains are dispatched from an industry or yard to the mainline. Trains are selected from the mainline and returned to industry or yard trackage.

You operate the mainline trains as the dispatcher of many trains, rather than an engineer of one train.

The automated mainline is divided into track "blocks." A train moves in one direction from a track block to a following track block.

Train control is simple. When a train just enters a block, the TBC determines if the "FOLLOWING" block is clear. If the "FOLLOWING" block is clear, the center rail for the block just entered is powered and the train proceeds to the "FOLLOWING" block. If the "FOLLOWING" block is occupied, center-rail power is withheld and the train coasts to a stop. When the "FOLLOWING" block clears, center-rail power is applied and the train proceeds.

The following diagrams show blocks "A," "B," and "C," with action detailed for block "B." All blocks are "coasting distance," or longer, have an outside rail insulated, and outside and center rails isolated from adjacent track blocks. Block "A" is "PREVIOUS" to block "B;" Block "C" "FOLLOWS" block "B."

A train in block "A" is approaching block "B."

The train enters block "B." The connection made by wheels and axles across the isolated outside-rail and the non-isolated outside-rail inform the controlling TBC that a train has entered its block.

The TBC determines that "FOLLOWING" block "C" is clear and activates its relay to supply power to the block "B" center-rail.

Block "A" remains powered. There is no interruption to the power to the train and lighted cars remain lit. Slow moving trains pass into block "B" without any hesitation.

The train clears block "A." The block "A" TBC deactivates its relay to remove power from the block and awaits another train.

This action is repeated for block "C" and all following blocks as the train moves through mainline track.

Again, a train just enters block "B." In this example, block "C" is occupied.

The TBC determines that "FOLLOWING" block "C" is occupied and does NOT activate its center-rail power relay.

Unpowered, the train coasts to a stop, remaining in block "B." The train must not coast into block "C" or it will be powered and run into the train occupying block "C."

The train in block "C" proceeds out of the block. Block "C," the block "FOLLOWING" block "B," is clear. The TBC for block "B" determines that its "FOLLOWING" block is clear, and activates its relay for block "B" center-rail power. The train is powered and proceeds as previously.

This same simple process is also applied to crossover and track switch elements. For a crossover, two track blocks are controlled, with only one of the two blocks allowed to be powered at a time. For track switches, two track blocks overlap, with only one track block allowed to be powered at a time. A TBC controlling a track switch performs additional functions of dispatching trains from or to a selected route and operating the track switch mechanism.

A "block" is a section of track that has one outside-rail insulated from (metal) ties and from the other outside-rail. The insulated outside- rail and the center-rail are isolated from adjacent track blocks. "Blocks" are longer than the "coasting distance" of any train.

Three-Rail Innovations' Train Block Controllers use the isolated outside-rail to detect when a train is in a track block. A relay controls train power to the isolated center rail of the block. Train power is applied to a block when the following block is clear; train power is withheld from a block when the next block occupied. A train stops when a block is not powered, then restarts when block power is applied. An engine must be set to start in a forward direction when power is applied to a block.

Practical design elements for an automated mainline layout are discussed following.

The block control system relies on a train coasting to a stop completely within a track block when train power is not applied to the block. If not, then the train is re-powered by the invaded block and will, likely, crash into the train ahead. Thus, ...

"Coasting distance" must be carefully determined as part of layout planning and track block locations for automatic train operation. Keep in mind that while separate track blocks can have different train power settings, all trains run at the same power settings for a given track block.

"Coasting distance" can be anything from a couple of feet for slow moving trolleys to eight feet or more for fast moving engines. "Coasting distance" is affected by:

• Engine type. Different engine types will have very different coasting distances.
• Train speed. Faster running means a longer coasting distance.
• Engine pickup location. Pickup-in-back will coast further than pickup-in-front. Determine coasting distance with pickup-in-back so a train can be placed on the track in any orientation.
• Consist. A train with a few cars will coast longer than the engine alone. Heavy cars will lengthen coasting distance.
• Running time. Trains run faster, and coast longer, after running for a half-hour or so.
• Track inclination. Downhill runs coast longer than flat runs.
• Lubrication. Recently lubricated trains will coast longer than lubrication-dry trains.

Determine "coasting distance" using example engines and consist to be run on the automatic layout, then add a safety factor.

A track "block" consists of track sections with one outside-rail and the center-rail isolated from adjacent track sections. Each end of the isolated rails has an insulating track pin, break in the rail, or other means for isolating outside and center rails from adjacent blocks. The isolated rails are contiguous within the block, so any interior pins in the isolated rails are steel (electrically conductive.)

One wire connection is made to the isolated outside-rail of each track block to a TBC outside-rail input terminal. This connection lets the TBC know when a train is in the track block.

A wire connection is also made from the isolated center-rail of the track block to a TBC relay terminal. The TBC uses a relay to activate train/center-rail power for a track block.

Supplemental wire connections can be made to blocks constructed from many track sections (say more than seven) so that block integrity does not depend upon numerous steel pins. In this case, the wires from supplemental connections are connected together and a single wire run to its TBC outside-rail input or relay terminals.

Track block construction is illustrated by the following diagram.

The isolation of an outside rail needs to consider other needs for rail isolation, such as non-derailing rails of O-22 (and similar) switches. The continuous connection of the non-isolated rail also needs to be considered; if the opposite outside rails of adjacent track sections are isolated, then the track common voltage on the non-isolated rails will be interrupted and an additional non-isolated rail power connection will need to be made.

Outside rails of switches, crossovers, and operating tracks are typically difficult to isolate, so block design needs to work around these track elements. Center-rails may also be connected internally, requiring special powering considerations.

The following track switch leg example is illustrative.

A track block is divided into a "leading" block and a "trailing" block which span the switch or crossover. The "leading" block must be "coasting distance" or longer. The "trailing" block is longer than the longest distance between wheel trucks of engines or rolling stock.

The Train Block Controller will work with any three-rail track system. The track system must be capable of:

• Insulation of one outside rail from metal ties and the other outside- rail.
• Isolation outside-rail and center-rail ends to form track blocks.
• Easy connection of wires to rails, preferably without the need for special track sections or visible connecting devices.

This web site uses high-rail tubular track for illustrations, but track blocks may be constructed with whatever track system is in use. Drawings show "insulated plastic pins" to represent an isolation break, implemented as required for a given track system. "Steel Pins" represent an electrically conductive connection.

Click the bar for details on constructing track blocks with various track systems.