Tilt Authorisation Speed Supervision

TASS Balise Class 221 TASS Indicator Panel

It has been designed jointly by Alstom Signalling and Alstom Transport and has been neatly developed as a first stage in the future ERTMS cab signalling.

The vital link between train and track is a passive balise (transponder) with no moving parts or wires.

The train computers work on the well-tried principle of a majority vote from two out of three computers, except for the odometers, where high precision speed recording is achieved through a vote from four out of six speed voting systems.

TASS Balise Data Information

TASS System

TASS Balise transmits data to the train including:

Distance to the next balise

Permissible Speed profiles and locations until next balise

Enhanced Permissible Speed profiles and locations until next balise

Braking Curve to safely operate at the reduced speed profiles

Tilt Authority locations and distances until the next balise

A Driver-Eye View of TASS

So far as drivers are concerned, TASS is a friend in the cab. If they are driving normally they will not be aware of its presence, as with TPWS. A typical train will depart Euston with Tilt Enabled, but still awaiting the command to actually start tilting. The train will encounter a Tilt Activate balise (a passive transponder in the track) soon after Primrose Hill Tunnel, which gives Tilt Authorisation and Speed Supervision in line with the infrastructure to the next balise. This authority is updated or refreshed by subsequent balises. To avoid the need to number balises, each balise will also tell the train where to expect the next one - usually in about 2 miles.

A tilt dis-enable balise is provided before areas of limited clearance and the train then tilt-locks in 7 seconds.

If the train does not find the next balise for any reason, TASS automatically dis-enables tilt and supervises a speed reduction to conventional speeds. Once the train has encountered two subsequent balises at the correct distances, TASS automatically re-enables tilt and permits tilt speeds.

Drivers operate the train in the usual way, but in areas where they are permitted to exceed the Permissible line speed, they will work to new Enhanced Permissible Speed (EPS). There will be separate EPS lineside signs.

85 mphEPS 100 mph

If the speed is exceeded by 3mph on any stretch of line, regardless of whether it is Permanent or an EPS speed, TASS will provide a warning to drivers to take corrective action. If the drivers fail to do this and the speed is exceeded by 6mph the brakes are automatically applied until the train speed has fallen to 20mph below the supervised line speed. They are then "offered control back".

Compare with C-APT Driver Aid System.

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© R G Latham 2024.