#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define X first
#define Y second
#define ALL(v) v.begin(), v.end()
#define SZ(a) ((int)a.size())
#define pb push_back

const int N = 2505;
const ll M = 1'000'000'007;

void add(ll &a, ll b) {
    a = (a + b) % M;
}

ll pw[N][N];
ll xs[N][N];
ll xsi[N][N];

ll ans;

vector<pll> v[N];

ll sz[N];
int n, m;

ll c[N][N];


void dfs(int now, int f) {
    sz[now] = 1; 
    for (auto [k, l] : v[now])
        if (k != f) {
            dfs(k, now);
            sz[now] += sz[k];
            ll s = sz[k];
            ll tmp = 0;
            for (int x = 0; x <= m; x++) {
                add(tmp, pw[s][x] * pw[n - s][m - x] % M * (m - x) % M * xs[x][s] % M * c[m][x] % M);
                add(tmp, pw[s][x] * pw[n - s][m - x] % M * xsi[x][s] % M * c[m][x] % M);
            }
            s = n - s;
            for (int x = 0; x <= m; x++) {
                add(tmp, pw[s][x] * pw[n - s][m - x] % M * (m - x) % M * xs[x][s] % M * c[m][x] % M);
                add(tmp, pw[s][x] * pw[n - s][m - x] % M * xsi[x][s] % M * c[m][x] % M);
            }
            for (int x = 0; x <= m; x++) {
                ll t = pw[s][x] * pw[n - s][x] % M * pw[s][m - x] % M * pw[n - s][m - x] % M * c[m][x] % M * c[m][x] % M; 
                t = t * m % M;
                add(tmp, -t);
            }
            add(ans, tmp * l % M);
        }
}

int main() {
    ios::sync_with_stdio(0), cin.tie(0);

    for (int i = 1; i < N; i++) {
        pw[i][0] = 1;
        for (int j = 1; j < N; j++)
            pw[i][j] = pw[i][j - 1] * i % M;
    }

    for (int i = 0; i < N; i++) {
        c[i][0] = c[i][i] = 1;
        for (int j = 1; j < i; j++)
            c[i][j] = (c[i - 1][j - 1] + c[i - 1][j]) % M;
    }


    cin >> n >> m;

    for (int x = 0; x <= n; x++)
        for (int s = 0; s <= n; s++) {
            xs[x][s] = pw[n - s][x] * pw[s][m - x] % M * c[m][x] % M;
            xsi[x][s] = pw[n - s][x] * pw[s][m - x] % M * x % M * c[m][x] % M;
        }
            
    for (int x = 1; x <= n; x++)
        for (int s = 0; s <= n; s++) {
            add(xs[x][s], xs[x - 1][s]);
            add(xsi[x][s], xsi[x - 1][s]);
        }

    for (int i = 1; i < n; i++) {
        ll a, b, c;
        cin >> a >> b >> c;
        v[a].pb({b, c});
        v[b].pb({a, c});
    }

    dfs(1, 1);
    cout << (ans + M) % M << endl; 
}