Existing technologies

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Gap fillers

Gap fillers are used to close only the horizontal gap at train stations.  However, these strips are hardly a comprehensive, or economical answer to independent accessibility and safety.  For the simple solution that it is, it sure costs a lot of money.  A train operator in Sydney recently spent $21M installing gap fillers at a cost of $450k per platform for what amounts to just a long strip of... rubber... Gap fillers don't eliminate steps, and there's very common and stubborn issues they can't solve. ​​Curved platforms and platform carve outs.

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Bridge plates and descending ramps

Sometimes, when there's enough space under the doors of the train and the vertical step up into the train isn't very high, there's the option to install a little automated ramp, built into the train, where a person can push a button from the outside or the inside of the train to cover the gap and allow them to board safely and independently.  Whilst that's virtually impossible for the train types, gaps and steps, etc, that we have in Victoria Australia, it can work in other places. The question then becomes - why aren't they everywhere now? One of the reasons is - those built in ramps require space below the door and some doors are directly above the bogies of the train car.  There's not enough space to retrofit them (like on Melbourne Metro and V/Line).  Descending ramps haven't been a more universal solution because the addition of those built-in ramps creates a single point of failure on the entire train line that wasn't there before.  A failure of that ramp can block the entire train line service until it can be repaired or removed.  Should that ramp fail to retract, a train with a ramp sticking out from under it's door cannot safety fit through tunnels and would sweep along the length of the platform like a knife blade.  The failure modes and effects of these designs are severe and this has prevented their widespread adoption by rail operators.

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​Ascending ramps

A few designs exist for ramps that are built into the platform of a train station. They only work on completely straight sections of platform and are the most economical when the trains they're used with can stop relatively accurately. They also can't be used in a mixed train type environment where the spacings between the doors of the train could change depending on the train that's arriving at the station. A curved platform with a train that cannot stop accurately is an impossible situation for the current designs of ascending ramps because the door will arrive a different angles relative to the ramp. In addition, some of the ascending ramps were designed to be modular as a means of compensating for trains that don't stop accurately, however this has very undesirable sizing, pricing, and reliability effects. Ramps that are built directly into the platform are fixed in place where persons living with disabilities must board and don't provide access to all doors of the train. It dictates where the train must stop, how accurately it must stop, and influences the minimum practical length of the train arriving at the station.  The ramp will typically be at the very end of the platform where the front of the train will stop.

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Communication based train control (CBTC) and High Capacity Signaling (HCS)

Is still a 'future' based technology that is in it's infancy relative to the speed at which new platforms and trains are constructed. CBTC with HCS is part of the answer to independent accessibility where both trains and tracks leading into platforms must be fitted with this new communication and signaling technology. New stations and new trains are typically built with the ability to stop a train within cm's of accuracy. This stopping accuracy allows for the construction of platform screens with automated doors that align to the train for the purpose of added safety. This improved accuracy can fix some, but not all, of the flaws of ascending ramp designs. Mixed train types aren't compatible with platform screen doors so accurate train stopping isn't going to help accessibility for those systems until the fleet of trains can be more uniform. It's going to be a long time before all stations and trains are fitted with CBTC to improve their stopping accuracy which limits the effectiveness of all ascending ramp designs. V/Line trains in regional Victoria cannot stop within 5m of their target, even in ideal weather conditions.