The DfT are more specific than that:
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Warning notices
4.21 Penalty fares warning notices must be displayed at every entrance to a CTA and at every penalty fares station so that at least one notice can be easily seen by anyone joining a penalty fares train (rule 4 of the Penalty Fares Rules). The best way to do this is to place a warning notice at every entrance to the station platforms. The notices must be prominent, easy to read, and easy to distinguish from other notices and the general surroundings. The notices must give the name of the operator, the circumstances in which a penalty fare may be charged and the amount of a penalty fare. To make sure that warning notices are consistent between different operators, they must be in line with a code of practice approved by the SRA. The Association of Train Operating Companies (ATOC) has produced a code of practice which we have approved for this purpose
4.22 The warning notices must be in place when the scheme is introduced, and must stay in place for as long as the scheme continues. If the notices at a particular station are covered up, damaged or removed for any reason, passengers who join a train at that station cannot be charged a penalty fare (rule 7). For this reason, the notices must be checked regularly and operators should include a check in their routine station audits.
4.23 As well as the fixed notices required by the rules, penalty fares warnings should be shown in the departure information for each penalty fares train shown on customer information screens and indicator boards at stations. For example, 'Please buy your ticket before boarding as penalty fares apply to this train'. This is especially important at larger stations, where penalty fares apply to certain trains but not others.
4.24 Another best practice is to display warning notices on trains, although this may not be possible if rolling stock is also used on routes where penalty fares do not apply.
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At Sheffield, EMT do not comply with the bits I have highlighted