Start with the rails already on the board
A familiar locomotive name can make a small boxed model look like a straightforward addition. For Hornby's TT3007M LNER Class A4 Mallard, the decisive first detail is TT:120. Hornby identifies TT as 1:120 scale on 12 mm gauge; its OO system is 1:76 on 16.5 mm gauge. An existing OO layout therefore is not the matching track system for this TT model.
Scale describes the model's size relationship to its subject. Gauge is the distance between the rails. Record both rather than using the word ‘small’ as a compatibility description.
The product code adds another condition
The TT3007M page specifies Radius 2 as the minimum curve. Choosing TT:120 track is therefore only the beginning: the actual route must also meet the locomotive's stated curve requirement. A collection of compatible straight sections does not establish that a tight oval will suit this particular engine.
DCC Ready is a separate description
The same page calls the locomotive DCC Ready with a Next18 socket. That wording identifies a preparation and socket, not a promise that a decoder is already fitted. Keep that question separate from whether the wheels meet the rails.
An old TT label needs its own answer
Hornby's FAQ distinguishes the current 1:120 range from older Tri-ang TT at 1:101.6 and says the two are not compatible. A second-hand ‘TT’ description without the actual range is therefore insufficient evidence for this purchase.
Before adding this Mallard, write down the exact locomotive code, the existing track system and its gauge, and the tightest documented curve. If the layout is OO, choose an appropriately documented OO locomotive instead; changing the name on the box cannot bridge the gauge difference.
Sources & further reading
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