In my experience, gateline assistants can stop people from travelling with what they consider 'invalid' tickets, therefore if they have authority to stop people travelling with invalid tickets, they also have authority to grant people access to the platforms.
I think its arguable on whether or not they can give you the authority to travel on a particular train though. I think that you can argue that if you passed through a barrier, and they inspected your ticket, you have authority to be at that station (if you are off route etc).
If they let you through to board a particular train (e.g. East Midland Trains at St Pancras) you can argue that they let you through to board that train with your advance ticket). I have experienced reservation checks at the barriers at Kings Cross before.
However, if they let you through the barriers at a generic station (e.g Clapham Junction) other than giving you authority to be at that particular station or via that particular station, they cannot be seen to be granting you authority to board any particular train.
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With regards to whether or not a person 'has particular authority' to grant passengers the right to be somewhere or not be somewhere, I would refer the reader to the case Canadian pacific railway company vs Lockhart:
Canadian Pacific Railway Company vs Lockhart said:
When considering the imposition of vicarious liability, ‘the first consideration is the ascertainment of what the servant is employed to do.’ (Lord Thankerton) and ‘It is clear that the master is responsible for acts actually authorised by him: for liability would exist in this case, even if the relation between the parties was merely one of agency, and not one of service at all.
But a master, as opposed to the employer of an independent contractor, is liable even for acts which he has not authorized, provided they are so connected with acts which he has authorized that they may rightly be regarded as modes – although improper modes – of doing them. In other words, a master is responsible not merely for what he authorizes his servant to do,but also for the way in which he does it . .
On the other hand, if the unauthorized and wrongful act of the servant is not so connected with the authorized act as to be a mode of doing it, but is an independent act, the master is not responsible: for in such a case, the servant is not acting in the course of his employment but has gone outside of it.’ and
‘It is often difficult in the particular case to distinguish between the second and the third of these situations, but the criterion is whether the act which is unauthorised is so connected with acts which have been authorised that it may be regarded as a mode – although an improper mode – of doing the authorised act, as distinct from constituting an independent act for which the master would not be liable.’
Lord Dunedin said: ‘there are prohibitions which limit the sphere of employment, and prohibitions which only deal with conduct within the sphere of employment.’
http://www.bailii.org/uk/cases/UKPC/1942/1942_24.pdf
I would argue that even if the gateline assistant was not specifically authorised to grant authority to passengers to travel, because this would be closely connected to his duties for the train operating company the train operating company would be liable for what its employee says.
Even if there was an absolute prohibition on him granting authority to passengers to travel on particular trains in his contract, that could still be challenged. His ability to grant authority to be on the platforms without a valid ticket would be unarguable though.
In this case, the train operating company would be liable for accepting the ticket as being valid.