Miniature plants for garden railways
Ground cover
Arenaria balearica (sandwort)
Calluna vulgaris ?Foxii Nana? (a tiny heather)
Frankenia laevis (bit invasive)
Mentha requiennii
Minuartia imbricato (invasive)
Potentilla eriocarpa
Raoulia (australis, glabra, lutescens) hard to grow ? good drainage required
Sagina boydii
Scleranthus biflorus (ideal miniature grass)
Soleirolia solerolii (?mind your own business? ? invasive)
Viola yakusimana (a tiny violet)
Shrubs
Arcterica nana (flowers ? likes some shade)
Berberis ?Corallina Compacta? (like a tiny holly bush)
Cotoneaster congestus nanus
Genista sagittalis pilosa minor (mini gorse)
Jasminum parkeri (mini jasmine ? slightly tender)
Rhododendron radicans (grows to only 4 inches)
Rubus articus (mini bramble ? largish leaves ? can be invasive)
Thymus ?Doone Valley? (looks like mini golden privet)
Tsusiophyllum tanakae (tiny formal shrub ? needs shade and peat)
Evergreen tree
Lonicera nitida ?baggesens gold?
Deciduous trees
Betula nana (mini birch tree)
Salix boydii (easy to grow)
Salix herbacea (creeping willow)
Salix hylematica
Salix myrsinites (mini weeping willow)
Salix retusa
Sorbus reducta (mini mountain ash)
Syringa velutina (palibiniana)[mini lilac]
Ulmus parvifolia (tiny elm)
Conifers
Abies balsamea hudsonia
Chamaecyparis obtusa ?nana caespitosa? (very small and slow growing)
? ? ?nana?(like mini oak tree)
? ? ?kosterii?
? ? ?pygmaea?
Chameacyparis pisifera ?nana?
? ? ?nana aureovariagata?
? ? ?plumosa compressa?
Crytomeria japonica vilmoriniana (like a mini monkey puzzle tree)
? ? Bandai sugi
Juniperus communis compressa (like a mini Lombardy poplar)
Juniperus coxii (weeping poplar)
Picea abies ?pygmaea?
Picea mariana ?nana?
Thuja orientalis ?minima?
I compiled the above list a few years ago from John Constable's book and recommendations from various Garden Rail a RM articles. It may be useful as a reference though I was too lazy to ever actually use it myself