// all station sprite sets are put in one single action1 set
spriteset (ground_tile, "") {
    []
}
spriteset (cargo_pile, "") {
    [] //empty
    [] //small pile of coal
    [] //larger pile of coal
    [] //very large pile
}

// I'm not sure about the extent to which passing parameters is already possible
spritelayout my_layout(amount) {
    ground {
        sprite: ground_tile;
    }
    building {
        sprite: cargo_pile(amount)
    }
}

switch(FEAT_STATIONS, SELF, coal_pile_amount, cargo_amount(COAL)) {
    0.. 50: my_layout(0);
    50.. 150: my_layout(1);
    150 .. 300: my_layout(2);
    my_layout(3);
}

switch(FEAT_STATIONS, SELF, coal_pile_cb, current_callback) {
    0x14: coal_pile_amount;
    CB_FAILED:
}

item(FEAT_STATIONS, coal_pile, 0) {
    property {
        class: "NML\0";
        //when defining the class, layouts are automatically added as well
    }
    graphics {
        //no cargo stuff here, do that via varaction2
        //automatically add a varaction2 that checks for CB 0 and branches to the action1/2 containing all sprites
        //i.e. 1 single action1/realaction2 per grf
        coal_pile_cb;
    }
}