Halliday & Matthiessen (2014: 606-7):
While conjunction, reference and substitution & ellipsis are cohesive resources within the grammatical zone of lexicogrammar, lexical cohesion operates within the lexical zone and is achieved through the choice of lexical items. Most typically, such cohesive relations hold between single lexical items, either words or larger units, e.g. locomotive (word), steam engine (group), in steam (phrase), steam up, get up steam (‘phrases’ in the dictionary sense); but also involving wordings having more than one lexical item in them, such as maintaining an express locomotive at full steam. … Lexical ties are independent of structure and may span long passages of intervening discourse …
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To be clear, lexical cohesion involves relations between single lexical items, whether single-worded, as in locomotive, or multi-worded, as in steam engine. It does not involve relations of hyponymy, meronymy etc. with multi-lexical item wordings such as maintaining an express locomotive at full steam. Cf. Halliday (1985: 314; 1994: 335):
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