Unanswered questions are mounting after a train crash that left one driver dead and nine people fighting for their lives in hospital in the worst rail disaster for 20 years. 

A total of 99 people were injured, 32 seriously, after a Luton Airport Express service running from Corby to St Pancras ploughed into the back of a Nottingham to St Pancras train at around 5.15pm on Friday.

It makes it the worst railway crash in Britain since the derailment of a train at Grayrigg, in Cumbria in 2007 - when one person was killed and 88 injured.

Passengers suffered broken bones and were left 'spitting out blood' when the two East Midlands Railway trains collided, after the Nottingham service ground to a halt on the tracks due to a technical fault.

Some travellers reported being 'flung' from their seats into the chairs and tables in front, before seeing the carriages fill with smoke – although the fire service has confirmed this was not the result of any blaze. 

A major incident was declared and more than 80 people received treatment in hospital last night, with 28 still in hospital on Saturday morning, Chief Constable Lucy D'Orsi of British Transport Police said. 

Brett Byatt, a teacher who was travelling on the Corby train which slammed into a stationary Nottingham carriage, said: 'I knew something was up because the train never slows down from Bedford to Luton - and I felt it brake.'

Amid reports of a technical fault with the first train's automatic warning system (AWS), passengers have hit out at the UK's ageing railway network.

The AWS system is designed to automatically apply the brakes if a driver fails to acknowledge an approaching red signal.

It is understood the Luton Airport Express train collided with the other stationary EMR service while its driver was reporting the issue to maintenance staff by phone.

Investigators and police remained at the scene, near Bedford, throughout Saturday as they began probing the possible cause of the crash. 

What was fault with automated warning system on the stopped train? 

Although it has been reported that there was a fault with the AWS on the stopped train, it is not known what this was or why it brought the train to a halt. 

Simon Calder, transport expert and former travel correspondent, said the Nottingham train had come to a halt 'because of a problem with the automatic warning system which warns train drivers if there is a red signal ahead'.

The driver of this train is understood to have been on the phone with maintenance staff to relay information of the fault when the Luton Express rammed into the back of it. 

It is not known if the second driver, who is the one believed to have died in the collision, had been informed there was a stationary train ahead or that there was a fault with the AWS. 

Due to confusion over the fault, after the crash staff on both trains leapt into action to activate a second warning system which ensured all signals in the area turned red, to avoid a further collision. 

Rail expert Tony Miles told the BBC that staff acted 'very quickly to protect the railway' by 'laying wires across the adjacent tracks' to trigger a secondary signalling system in case the first one had failed.

Was there a red signal, and if so why did the driver cross it? 

In his interview, Mr Miles, from Modern Railways magazine, said authorities must establish whether the signals in the area were on red and the Luton Express train from Corby had passed them anyway.

'Were the signals showing red and the train went past them, or were the signals showing that the line was clear and, if so, how were they able to know when the train in front was stopped?' he said.

He continued: 'As long as the track is occupied, it shouldn't be possible to clear the signals.'

Mr Miles said that while a driver could theoretically make a decision to cross a red signal, 'they would normally only do that because they had been given permission by a signal box because there was a fault'.

Investigators will be looking at whether the stopped train and AWS fault had led to a red signal in the path of the second train.

If there was, attention will no doubt turn to whether the driver saw and crossed the signal anyway, and if he was assured he had permission to do so before taking that decision.

Did the driver make a mistake, or could he have been taken ill? 

As the circumstances of the collision are unknown, investigators will be sure to consider whether a mistake was made, or if the second driver was taken ill in some way. 

Images from the scene appear to show that the collision was not at high speed, which would corroborate passengers' reports on the Corby train that they felt the service brake before the collision.

This could indicate they had seen the train and attempted to avert disaster, or that something else had gone wrong. Even if the driver did brake, experts have pointed out that the weight of passenger trains mean they take a long time to slow down, and can still pose a significant danger on impact even if only travelling very slowly.

Mr Miles told Sky News: 'The question has to be how has that train that's in the rear got into contact with the train that it was following.

'Obviously it's either gone past the signal that was telling it it should stop, or the signal was faulty, or the driver's made a mistake in some way, or didn't read the signal, or something.'

Although a mistake is possible, experts have pointed to advanced signalling and warning systems which should have prevented the second train ever crashing into the stationary one - assuming the message it had stopped had been passed along correctly.

The driver of the Luton Express could have been trying to slow the train and prevent the crash in the moments before the tragedy - or they could have had a medical incident such as a heart attack or stroke.

There is no evidence at this stage that the driver of the moving train was taken ill, and investigations are still in their early stages.. 

Will the trains' 'black boxes' reveal what happened? 

A railway expert said today that investigators would 'already know' what happened in the Bedford rail crash from 'second by second' airline-style black box recorders in the trains.

Mr Miles said they recorded 'every switch that was pressed and every control that was activated' in both trains.

The UK's train network is generally considered to be one of the safest in the world, and each is fitted with a 'black box' that tracks its location, movements and controls.

Some details, such as the locations from and times the trains set off, are already known.

The first train left Nottingham for London St Pancras at 3.50pm, while the second - the Luton Express - left Corby for St Pancras at 4.40pm.

Both were heading in the same direction and to the same destination. 

'They will know every switch that was pressed,' Mr Miles said.

'They will know every control that was activated in both of the trains and they will have second by second data from the trains as to what was going on.

'So I suspect there are probably people who have got a reasonable insight into what happened already but it doesn't mean they will release it.'

Did the moving train have permission to cross points to get to the other line?

Investigators looking at how the two trains could physically have collided will finally be examining whether the second train had been given permission to cross points to get onto the same line as the stationary Nottingham service.

Mr Miles said that the route of the trains, which collided near Elstow, close to Bedford, meant the Luton service would have had to cross multiple sets of points to get onto the same line as the first train.

It would have been 'going quite slowly as it was crossing a lot of points to get to the other line', he said, so the question would be: 'was it cleared to do that?'.

'The drivers would stop at a red signal regardless and talk on their radio,' he added. 

'So the two questions will be "were the signals showing red and the train went past them?" or "has something catastrophically gone wrong with the electronics system that interlocks itself so it can't allow a dangerous situation to happen?", and this then allowed the train to go past.